diff --git a/hcie-iced-app/crates/hcie-iced-gui/src/app.rs b/hcie-iced-app/crates/hcie-iced-gui/src/app.rs
index feba3e0..794c5ab 100644
--- a/hcie-iced-app/crates/hcie-iced-gui/src/app.rs
+++ b/hcie-iced-app/crates/hcie-iced-gui/src/app.rs
@@ -177,6 +177,10 @@ pub struct HcieIcedApp {
     pub ai_script_model: String,
     /// AI script generator — base URL for the selected provider.
     pub ai_script_url: String,
+    /// Whether the FastStone-like image viewer is active.
+    pub viewer_active: bool,
+    /// Viewer state for directory browsing and thumbnail grid.
+    pub viewer_state: crate::viewer::ViewerState,
 }
 
 /// A recently opened file entry.
@@ -507,6 +511,14 @@ pub enum Message {
 
     // ── Misc ────────────────────────────────────────────
     NoOp,
+
+    // ── Image Viewer ────────────────────────────────────
+    ViewerToggle,
+    ViewerNavigate(std::path::PathBuf),
+    ViewerSelectFile(std::path::PathBuf),
+    ViewerOpenFile(std::path::PathBuf),
+    ViewerRefresh,
+    ViewerFullscreen(bool),
 }
 
 /// Convert a FilterType enum to the engine's string filter ID.
@@ -674,6 +686,8 @@ impl HcieIcedApp {
             ai_script_provider: "ollama".to_string(),
             ai_script_model: "qwen2.5:7b".to_string(),
             ai_script_url: "http://localhost:11434".to_string(),
+            viewer_active: false,
+            viewer_state: crate::viewer::ViewerState::default(),
         };
 
         // Apply saved settings to tool state
@@ -3257,6 +3271,86 @@ impl HcieIcedApp {
                 self.window_id = Some(id);
             }
 
+            // ── Image Viewer ─────────────────────────────────
+            Message::ViewerToggle => {
+                self.viewer_active = !self.viewer_active;
+                if self.viewer_active && self.viewer_state.current_dir.as_os_str().is_empty() {
+                    // Initialize to home directory on first open
+                    let start_dir = dirs::home_dir()
+                        .unwrap_or_else(|| std::path::PathBuf::from("."));
+                    self.viewer_state = crate::viewer::ViewerState::new(start_dir);
+                }
+                if self.viewer_active {
+                    self.viewer_state.load_preview();
+                }
+            }
+            Message::ViewerNavigate(path) => {
+                self.viewer_state.set_dir(path);
+                self.viewer_state.load_preview();
+            }
+            Message::ViewerSelectFile(path) => {
+                // Select the file in the grid
+                if let Some(idx) = self.viewer_state.images.iter().position(|p| p == &path) {
+                    self.viewer_state.active_image_idx = idx;
+                    self.viewer_state.load_preview();
+                    self.viewer_state.view_mode = crate::viewer::ViewerMode::Viewer;
+                }
+            }
+            Message::ViewerOpenFile(path) => {
+                // Open the file in the editor — exit viewer and open the file
+                self.viewer_active = false;
+                let path_str = path.to_string_lossy().to_string();
+                let ext = path.extension()
+                    .and_then(|e| e.to_str())
+                    .unwrap_or("")
+                    .to_lowercase();
+                let result = match ext.as_str() {
+                    "psd" => self.documents[0].engine.import_psd(&path_str),
+                    "kra" => self.documents[0].engine.import_kra(&path_str),
+                    "hcie" => self.documents[0].engine.load_native(&path_str),
+                    _ => self.documents[0].engine.open_image(&path_str),
+                };
+                match result {
+                    Ok(()) => {
+                        self.documents[0].engine.pre_tile_all_layers();
+                        self.documents[0].name = path.file_name()
+                            .map(|n| n.to_string_lossy().to_string())
+                            .unwrap_or_else(|| "Untitled".to_string());
+                        self.documents[0].source_path = Some(path.clone());
+                        self.add_recent_file(&path);
+                        let composite = self.documents[0].engine.get_composite_pixels();
+                        self.documents[0].composite_raw = composite;
+                        self.documents[0].composite_pixels = std::sync::Arc::new(self.documents[0].composite_raw.clone());
+                        self.documents[0].full_upload.replace(true);
+                        self.documents[0].render_generation = self.documents[0].render_generation.wrapping_add(1);
+                        self.documents[0].cached_layers = self.documents[0].engine.layer_infos();
+                        let history_len = self.documents[0].engine.history_len();
+                        self.documents[0].cached_history = (0..history_len)
+                            .filter_map(|i| self.documents[0].engine.history_description(i).map(|d| (i, d)))
+                            .collect();
+                        self.documents[0].history_current = self.documents[0].engine.history_current();
+                        return Task::perform(async {}, |_| Message::CompositeRefresh);
+                    }
+                    Err(e) => {
+                        log::error!("Failed to open file from viewer: {}", e);
+                    }
+                }
+            }
+            Message::ViewerRefresh => {
+                self.viewer_state.clear_caches();
+                self.viewer_state.load_preview();
+            }
+            Message::ViewerFullscreen(fs) => {
+                self.viewer_state.fullscreen = fs;
+                if let Some(id) = self.window_id {
+                    if fs {
+                        return iced::window::change_level(id, iced::window::Level::AlwaysOnTop);
+                    } else {
+                        return iced::window::change_level(id, iced::window::Level::Normal);
+                    }
+                }
+            }
+
             Message::NoOp => {}
         }
 
@@ -3271,9 +3365,23 @@ impl HcieIcedApp {
     /// dock grid, and status bar surround the content. Dialogs overlay everything.
     pub fn view(&self) -> Element<'_, Message> {
         let view_start = std::time::Instant::now();
-        let doc = &self.documents[self.active_doc];
         let colors = self.theme_state.colors();
 
+        // When the viewer is active, show the full-screen image viewer
+        if self.viewer_active {
+            let viewer_panel = crate::viewer::view(&self.viewer_state, colors);
+            return container(viewer_panel)
+                .width(Length::Fill)
+                .height(Length::Fill)
+                .style(move |_theme| iced::widget::container::Style {
+                    background: Some(iced::Background::Color(colors.bg_app)),
+                    ..Default::default()
+                })
+                .into();
+        }
+
+        let doc = &self.documents[self.active_doc];
+
         // Title bar (now includes menu items and window controls)
         let title_bar = panels::title_bar::view(
             &doc.name,
diff --git a/hcie-iced-app/crates/hcie-iced-gui/src/main.rs b/hcie-iced-app/crates/hcie-iced-gui/src/main.rs
index 86ee626..7d43399 100644
--- a/hcie-iced-app/crates/hcie-iced-gui/src/main.rs
+++ b/hcie-iced-app/crates/hcie-iced-gui/src/main.rs
@@ -16,6 +16,7 @@ mod selection;
 mod settings;
 mod sidebar;
 mod theme;
+mod viewer;
 
 use app::HcieIcedApp;
 
diff --git a/hcie-iced-app/crates/hcie-iced-gui/src/viewer/directory_tree.rs b/hcie-iced-app/crates/hcie-iced-gui/src/viewer/directory_tree.rs
new file mode 100644
index 0000000..7eda194
--- /dev/null
+++ b/hcie-iced-app/crates/hcie-iced-gui/src/viewer/directory_tree.rs
@@ -0,0 +1,198 @@
+//! Directory tree navigation panel.
+//!
+//! Displays a collapsible tree of filesystem directories with standard
+//! locations (Home, Desktop, Documents, Downloads, Pictures, Videos, Root)
+//! as top-level roots. Each node shows its subdirectories when expanded.
+
+use crate::app::Message;
+use crate::theme::ThemeColors;
+use crate::viewer::ViewerState;
+use iced::widget::{button, column, container, scrollable, text};
+use iced::{Element, Length};
+use std::path::{Path, PathBuf};
+
+/// A root directory entry with display name, path, and icon.
+struct FolderRoot {
+    name: String,
+    path: PathBuf,
+    icon: &'static str,
+}
+
+/// Get the standard directory roots for the tree.
+fn get_roots() -> Vec<FolderRoot> {
+    let mut roots = Vec::new();
+    if let Some(home) = dirs::home_dir() {
+        roots.push(FolderRoot {
+            name: "Home".to_string(),
+            path: home,
+            icon: "~",
+        });
+    }
+    if let Some(desktop) = dirs::desktop_dir() {
+        roots.push(FolderRoot {
+            name: "Desktop".to_string(),
+            path: desktop,
+            icon: "D",
+        });
+    }
+    if let Some(docs) = dirs::document_dir() {
+        roots.push(FolderRoot {
+            name: "Documents".to_string(),
+            path: docs,
+            icon: "Docs",
+        });
+    }
+    if let Some(downloads) = dirs::download_dir() {
+        roots.push(FolderRoot {
+            name: "Downloads".to_string(),
+            path: downloads,
+            icon: "DL",
+        });
+    }
+    if let Some(pics) = dirs::picture_dir() {
+        roots.push(FolderRoot {
+            name: "Pictures".to_string(),
+            path: pics,
+            icon: "Pic",
+        });
+    }
+    if let Some(vids) = dirs::video_dir() {
+        roots.push(FolderRoot {
+            name: "Videos".to_string(),
+            path: vids,
+            icon: "Vid",
+        });
+    }
+    let root_path = PathBuf::from("/");
+    if root_path.exists() {
+        roots.push(FolderRoot {
+            name: "Root (/)".to_string(),
+            path: root_path,
+            icon: "/",
+        });
+    }
+    roots
+}
+
+/// Scale a color's alpha by a factor (simulates gamma_multiply).
+fn color_alpha(color: iced::Color, alpha: f32) -> iced::Color {
+    iced::Color {
+        a: color.a * alpha,
+        ..color
+    }
+}
+
+/// Build the directory tree view.
+pub fn view(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    let roots = get_roots();
+    let mut items = column![].spacing(2);
+
+    for root in &roots {
+        let node = draw_tree_node(&root.path, &root.name, root.icon, state, colors, 0);
+        items = items.push(node);
+    }
+
+    scrollable(items.width(Length::Fill))
+        .height(Length::Fill)
+        .into()
+}
+
+/// Recursively draw a tree node with its children.
+fn draw_tree_node(
+    path: &Path,
+    name: &str,
+    icon: &str,
+    state: &ViewerState,
+    colors: ThemeColors,
+    depth: usize,
+) -> Element<'static, Message> {
+    let is_selected = state.current_dir == path;
+    let indent = depth as f32 * 16.0;
+
+    let subdirs = get_subdirs_for_path(path);
+    let has_children = !subdirs.is_empty();
+
+    let label = format!("{} {}", icon, name);
+    let label_color = if is_selected {
+        colors.accent
+    } else {
+        colors.text_primary
+    };
+
+    let label_text = text(label.clone())
+        .size(12)
+        .style(move |_theme| iced::widget::text::Style {
+            color: Some(label_color),
+        });
+
+    let row_content: Element<'static, Message> = if has_children {
+        let mut children_col = column![].spacing(1);
+        for child in &subdirs {
+            if let Some(child_name) = child.file_name().and_then(|n| n.to_str()) {
+                let child_node =
+                    draw_tree_node(child, child_name, ">", state, colors, depth + 1);
+                children_col = children_col.push(child_node);
+            }
+        }
+
+        let btn = button(label_text)
+            .on_press(Message::ViewerNavigate(path.to_path_buf()))
+            .width(Length::Fill)
+            .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+                iced::widget::button::Style {
+                    background: Some(iced::Background::Color(if is_selected {
+                        color_alpha(colors.accent, 0.15)
+                    } else {
+                        iced::Color::TRANSPARENT
+                    })),
+                    text_color: label_color,
+                    border: iced::Border::default(),
+                    ..Default::default()
+                }
+            });
+
+        column![btn, children_col].spacing(1).into()
+    } else {
+        button(label_text)
+            .on_press(Message::ViewerNavigate(path.to_path_buf()))
+            .width(Length::Fill)
+            .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+                iced::widget::button::Style {
+                    background: Some(iced::Background::Color(if is_selected {
+                        color_alpha(colors.accent, 0.15)
+                    } else {
+                        iced::Color::TRANSPARENT
+                    })),
+                    text_color: label_color,
+                    border: iced::Border::default(),
+                    ..Default::default()
+                }
+            })
+            .into()
+    };
+
+    container(row_content)
+        .width(Length::Fill)
+        .padding(iced::Padding {
+            top: 0.0,
+            right: 0.0,
+            bottom: 0.0,
+            left: indent,
+        })
+        .into()
+}
+
+/// Get subdirectories of a path (non-cached version for the view).
+fn get_subdirs_for_path(path: &Path) -> Vec<PathBuf> {
+    let mut dirs = Vec::new();
+    if let Ok(entries) = std::fs::read_dir(path) {
+        for entry in entries.flatten() {
+            let p = entry.path();
+            if p.is_dir() {
+                dirs.push(p);
+            }
+        }
+    }
+    dirs.sort();
+    dirs
+}
diff --git a/hcie-iced-app/crates/hcie-iced-gui/src/viewer/mod.rs b/hcie-iced-app/crates/hcie-iced-gui/src/viewer/mod.rs
new file mode 100644
index 0000000..b7d0b40
--- /dev/null
+++ b/hcie-iced-app/crates/hcie-iced-gui/src/viewer/mod.rs
@@ -0,0 +1,674 @@
+//! FastStone-like Image Viewer Mode implementation.
+//!
+//! Provides a full-screen image browser with:
+//! - Left sidebar: directory tree navigation + preview panel
+//! - Right panel: thumbnail grid with lazy loading
+//! - Keyboard navigation (arrow keys, Enter)
+//! - Fullscreen toggle
+//! - Browser and Viewer modes
+//!
+//! Ported from the egui version at `hcie-gui-egui/src/app/shell/viewer.rs`.
+
+pub mod directory_tree;
+pub mod thumbnail_grid;
+
+use crate::app::Message;
+use crate::theme::ThemeColors;
+use iced::widget::{button, column, container, horizontal_rule, row, scrollable, text, Space};
+use iced::{Element, Length};
+use std::collections::{HashMap, HashSet};
+use std::path::{Path, PathBuf};
+
+/// Supported image file extensions for the viewer.
+const SUPPORTED_EXTENSIONS: &[&str] = &[
+    "png", "jpg", "jpeg", "webp", "bmp", "gif", "tiff", "tif", "avif", "ico", "pnm", "qoi", "ff",
+    "hdr", "dds", "tga", "exr",
+];
+
+/// Viewer display mode — Browser shows the dual-pane directory view,
+/// Viewer shows a single image with navigation arrows.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum ViewerMode {
+    #[default]
+    Browser,
+    Viewer,
+}
+
+/// Persistent viewer state — directory listing, thumbnail cache, zoom/pan.
+pub struct ViewerState {
+    /// Currently browsed directory.
+    pub current_dir: PathBuf,
+    /// Supported image files in the current directory.
+    pub images: Vec<PathBuf>,
+    /// Index of the currently selected/active image.
+    pub active_image_idx: usize,
+    /// Current display mode.
+    pub view_mode: ViewerMode,
+    /// Whether the viewer is in fullscreen mode.
+    pub fullscreen: bool,
+    /// Cached subdirectory lists to avoid repeated disk reads.
+    pub subdirs_cache: HashMap<PathBuf, Vec<PathBuf>>,
+    /// Cached thumbnail RGBA pixel data (path → pixels, width, height).
+    pub thumbnail_cache: HashMap<PathBuf, (Vec<u8>, u32, u32)>,
+    /// Paths that failed to load as thumbnails.
+    pub failed_thumbnails: HashSet<PathBuf>,
+    /// Preview image RGBA pixel data and dimensions.
+    pub preview_pixels: Option<(Vec<u8>, u32, u32)>,
+    /// Preview load error message.
+    pub load_error: Option<String>,
+    /// Zoom level for the viewer panel.
+    pub zoom: f32,
+    /// Pan offset for the viewer panel.
+    pub pan_offset: (f32, f32),
+}
+
+impl Default for ViewerState {
+    fn default() -> Self {
+        Self {
+            current_dir: PathBuf::new(),
+            images: Vec::new(),
+            active_image_idx: 0,
+            view_mode: ViewerMode::Browser,
+            fullscreen: false,
+            subdirs_cache: HashMap::new(),
+            thumbnail_cache: HashMap::new(),
+            failed_thumbnails: HashSet::new(),
+            preview_pixels: None,
+            load_error: None,
+            zoom: 1.0,
+            pan_offset: (0.0, 0.0),
+        }
+    }
+}
+
+impl ViewerState {
+    /// Create a new viewer state starting at the specified directory.
+    pub fn new(start_dir: PathBuf) -> Self {
+        log::debug!(
+            "Initializing ViewerState with starting directory: {}",
+            start_dir.display()
+        );
+        let mut state = Self {
+            current_dir: start_dir,
+            ..Default::default()
+        };
+        state.refresh_images();
+        state
+    }
+
+    /// Refresh the images list in the current directory.
+    pub fn refresh_images(&mut self) {
+        log::debug!("Refreshing image list for: {}", self.current_dir.display());
+        self.images.clear();
+        if let Ok(entries) = std::fs::read_dir(&self.current_dir) {
+            for entry in entries.flatten() {
+                let path = entry.path();
+                if path.is_file() && is_supported_image(&path) {
+                    self.images.push(path);
+                }
+            }
+        }
+        self.images.sort();
+        self.active_image_idx = 0;
+        self.preview_pixels = None;
+        self.load_error = None;
+        log::debug!("Found {} supported images", self.images.len());
+    }
+
+    /// Move selection to the next image.
+    pub fn next_image(&mut self) {
+        if !self.images.is_empty() {
+            self.active_image_idx = (self.active_image_idx + 1) % self.images.len();
+            self.preview_pixels = None;
+            self.load_error = None;
+            self.zoom = 1.0;
+            self.pan_offset = (0.0, 0.0);
+            log::trace!("Navigated to next image, index: {}", self.active_image_idx);
+        }
+    }
+
+    /// Move selection to the previous image.
+    pub fn prev_image(&mut self) {
+        if !self.images.is_empty() {
+            self.active_image_idx = if self.active_image_idx == 0 {
+                self.images.len() - 1
+            } else {
+                self.active_image_idx - 1
+            };
+            self.preview_pixels = None;
+            self.load_error = None;
+            self.zoom = 1.0;
+            self.pan_offset = (0.0, 0.0);
+            log::trace!(
+                "Navigated to previous image, index: {}",
+                self.active_image_idx
+            );
+        }
+    }
+
+    /// Change active directory and clear thumbnail caches.
+    pub fn set_dir(&mut self, path: PathBuf) {
+        log::debug!("Setting current directory to: {}", path.display());
+        self.current_dir = path;
+        self.thumbnail_cache.clear();
+        self.failed_thumbnails.clear();
+        self.refresh_images();
+    }
+
+    /// Clear all file system and folder caches.
+    pub fn clear_caches(&mut self) {
+        log::debug!("Clearing directory and thumbnail caches");
+        self.subdirs_cache.clear();
+        self.thumbnail_cache.clear();
+        self.failed_thumbnails.clear();
+        self.refresh_images();
+    }
+
+    /// Get subdirectories of a path, cached to avoid repeated disk reads.
+    pub fn get_subdirs(&mut self, path: &Path) -> &[PathBuf] {
+        if !self.subdirs_cache.contains_key(path) {
+            let mut dirs = Vec::new();
+            if let Ok(entries) = std::fs::read_dir(path) {
+                for entry in entries.flatten() {
+                    let p = entry.path();
+                    if p.is_dir() {
+                        dirs.push(p);
+                    }
+                }
+            }
+            dirs.sort();
+            self.subdirs_cache.insert(path.to_path_buf(), dirs);
+        }
+        self.subdirs_cache.get(path).unwrap()
+    }
+
+    /// Load the full preview image for the currently selected image.
+    pub fn load_preview(&mut self) {
+        if self.images.is_empty() {
+            return;
+        }
+        let path = &self.images[self.active_image_idx];
+        log::debug!("Loading full preview image: {}", path.display());
+        match image::open(path) {
+            Ok(dyn_img) => {
+                let rgba = dyn_img.to_rgba8();
+                let w = rgba.width();
+                let h = rgba.height();
+                let pixels = rgba.into_raw();
+                self.preview_pixels = Some((pixels, w, h));
+                self.load_error = None;
+            }
+            Err(e) => {
+                let msg = format!("{}", e);
+                log::warn!("Failed to load preview {}: {}", path.display(), msg);
+                self.preview_pixels = None;
+                self.load_error = Some(msg);
+            }
+        }
+    }
+
+    /// Load a thumbnail for the given image path (120x120 max).
+    pub fn load_thumbnail(&mut self, path: &Path) {
+        if self.thumbnail_cache.contains_key(path) || self.failed_thumbnails.contains(path) {
+            return;
+        }
+        log::debug!("Lazy loading thumbnail for: {}", path.display());
+        match image::open(path) {
+            Ok(dyn_img) => {
+                let thumb = dyn_img.thumbnail(120, 120);
+                let rgba = thumb.to_rgba8();
+                let w = rgba.width();
+                let h = rgba.height();
+                let pixels = rgba.into_raw();
+                self.thumbnail_cache.insert(path.to_path_buf(), (pixels, w, h));
+            }
+            Err(_) => {
+                self.failed_thumbnails.insert(path.to_path_buf());
+            }
+        }
+    }
+}
+
+/// Check if a file path has a supported image extension.
+pub fn is_supported_image(path: &Path) -> bool {
+    path.extension()
+        .and_then(|ext| ext.to_str())
+        .map(|ext| SUPPORTED_EXTENSIONS.contains(&ext.to_lowercase().as_str()))
+        .unwrap_or(false)
+}
+
+/// Fit a texture size into available space while preserving aspect ratio.
+fn _fit_size(texture_w: f32, texture_h: f32, avail_w: f32, avail_h: f32) -> (f32, f32) {
+    if texture_w <= 0.0 || texture_h <= 0.0 || avail_w <= 0.0 || avail_h <= 0.0 {
+        return (avail_w, avail_h);
+    }
+    let scale = (avail_w / texture_w).min(avail_h / texture_h);
+    (texture_w * scale, texture_h * scale)
+}
+
+/// Build the viewer panel UI element.
+pub fn view(
+    state: &ViewerState,
+    colors: ThemeColors,
+) -> Element<'static, Message> {
+    let header = view_header(state, colors);
+    let body = match state.view_mode {
+        ViewerMode::Browser => view_browser(state, colors),
+        ViewerMode::Viewer => view_viewer(state, colors),
+    };
+    column![header, horizontal_rule(1), body]
+        .width(Length::Fill)
+        .height(Length::Fill)
+        .into()
+}
+
+/// Header toolbar with mode toggle, fullscreen toggle, and exit button.
+fn view_header(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    let exit_btn = button(text("  Exit Viewer  "))
+        .on_press(Message::ViewerToggle)
+        .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+            iced::widget::button::Style {
+                background: Some(iced::Background::Color(colors.accent)),
+                text_color: iced::Color::WHITE,
+                border: iced::Border::default().color(colors.accent).width(1),
+                ..Default::default()
+            }
+        });
+
+    let fs_label = if state.fullscreen { "Windowed" } else { "Fullscreen" };
+    let fs_btn = button(text(format!("  {}  ", fs_label)))
+        .on_press(Message::ViewerFullscreen(!state.fullscreen))
+        .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+            iced::widget::button::Style {
+                background: Some(iced::Background::Color(colors.bg_panel)),
+                text_color: colors.text_primary,
+                border: iced::Border::default().color(colors.border_low).width(1),
+                ..Default::default()
+            }
+        });
+
+    let is_browser = state.view_mode == ViewerMode::Browser;
+    let browser_btn = button(text("  Browse  "))
+        .on_press(Message::ViewerNavigate(state.current_dir.clone()))
+        .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+            iced::widget::button::Style {
+                background: Some(iced::Background::Color(if is_browser {
+                    colors.accent
+                } else {
+                    colors.bg_panel
+                })),
+                text_color: if is_browser {
+                    iced::Color::WHITE
+                } else {
+                    colors.text_primary
+                },
+                border: iced::Border::default().color(colors.border_low).width(1),
+                ..Default::default()
+            }
+        });
+
+    let is_viewer = state.view_mode == ViewerMode::Viewer;
+    let viewer_btn = button(text("  Viewer  "))
+        .on_press(Message::ViewerNavigate(state.current_dir.clone()))
+        .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+            iced::widget::button::Style {
+                background: Some(iced::Background::Color(if is_viewer {
+                    colors.accent
+                } else {
+                    colors.bg_panel
+                })),
+                text_color: if is_viewer {
+                    iced::Color::WHITE
+                } else {
+                    colors.text_primary
+                },
+                border: iced::Border::default().color(colors.border_low).width(1),
+                ..Default::default()
+            }
+        });
+
+    let refresh_btn = button(text("  Refresh  "))
+        .on_press(Message::ViewerRefresh)
+        .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+            iced::widget::button::Style {
+                background: Some(iced::Background::Color(colors.bg_panel)),
+                text_color: colors.text_primary,
+                border: iced::Border::default().color(colors.border_low).width(1),
+                ..Default::default()
+            }
+        });
+
+    row![
+        exit_btn,
+        text("  "),
+        fs_btn,
+        text("  "),
+        browser_btn,
+        viewer_btn,
+        text("  "),
+        refresh_btn,
+    ]
+    .align_y(iced::Alignment::Center)
+    .width(Length::Fill)
+    .into()
+}
+
+/// Browser mode — dual-pane layout with directory tree + preview on left,
+/// thumbnail grid on right.
+fn view_browser(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    let left_sidebar = view_left_sidebar(state, colors);
+    let right_panel = view_right_panel(state, colors);
+
+    row![left_sidebar, right_panel]
+        .width(Length::Fill)
+        .height(Length::Fill)
+        .spacing(4)
+        .into()
+}
+
+/// Left sidebar: folder tree on top, preview panel on bottom.
+fn view_left_sidebar(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    let tree_panel = {
+        let tree_content = directory_tree::view(state, colors);
+        container(
+            column![
+                text("Folders").style(move |_theme| iced::widget::text::Style {
+                    color: Some(colors.text_primary),
+                }),
+                horizontal_rule(1),
+                tree_content,
+            ]
+            .spacing(4)
+            .width(Length::Fill)
+            .height(Length::Fill),
+        )
+        .width(280)
+        .height(Length::Fill)
+        .style(move |_theme| iced::widget::container::Style {
+            background: Some(iced::Background::Color(colors.bg_panel)),
+            border: iced::Border::default()
+                .color(colors.border_low)
+                .width(1)
+                .rounded(8),
+            ..Default::default()
+        })
+    };
+
+    let preview_panel = view_preview(state, colors);
+
+    column![tree_panel, preview_panel]
+        .width(280)
+        .height(Length::Fill)
+        .spacing(4)
+        .into()
+}
+
+/// Preview panel showing the selected image thumbnail and metadata.
+fn view_preview(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    let content = if let Some((pixels, w, h)) = &state.preview_pixels {
+        let img = iced::widget::Image::new(iced::widget::image::Handle::from_rgba(
+            *w, *h, pixels.clone(),
+        ))
+        .width(Length::Fill)
+        .height(Length::Shrink);
+
+        let filename = state
+            .images
+            .get(state.active_image_idx)
+            .and_then(|p| p.file_name())
+            .map(|n| n.to_string_lossy().to_string())
+            .unwrap_or_default();
+
+        let dim_text = format!("{}x{}", w, h);
+
+        column![
+            img,
+            text(filename.clone())
+                .size(11)
+                .style(move |_theme| iced::widget::text::Style {
+                    color: Some(colors.text_primary),
+                }),
+            text(dim_text.clone())
+                .size(10)
+                .style(move |_theme| iced::widget::text::Style {
+                    color: Some(colors.text_secondary),
+                }),
+        ]
+        .spacing(4)
+        .align_x(iced::Alignment::Center)
+        .width(Length::Fill)
+    } else if let Some(err) = &state.load_error {
+        column![text(format!("Error: {}", err)).style(move |_theme| iced::widget::text::Style {
+            color: Some(colors.danger),
+        })]
+        .align_x(iced::Alignment::Center)
+        .width(Length::Fill)
+    } else {
+        column![text("No image selected").style(move |_theme| iced::widget::text::Style {
+            color: Some(colors.text_secondary),
+        })]
+        .align_x(iced::Alignment::Center)
+        .width(Length::Fill)
+    };
+
+    container(
+        column![
+            text("Preview").style(move |_theme| iced::widget::text::Style {
+                color: Some(colors.text_primary),
+            }),
+            horizontal_rule(1),
+            content,
+        ]
+        .spacing(4)
+        .width(Length::Fill)
+        .height(Length::Fill),
+    )
+    .height(220)
+    .width(280)
+    .style(move |_theme| iced::widget::container::Style {
+        background: Some(iced::Background::Color(colors.bg_panel)),
+        border: iced::Border::default()
+            .color(colors.border_low)
+            .width(1)
+            .rounded(8),
+        ..Default::default()
+    })
+    .into()
+}
+
+/// Right panel: toolbar with path, stats, and thumbnail grid.
+fn view_right_panel(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    let has_parent = state.current_dir.parent().is_some();
+
+    let parent_btn = if has_parent {
+        button(text("  Parent  "))
+            .on_press(Message::ViewerNavigate(
+                state.current_dir.parent().unwrap().to_path_buf(),
+            ))
+    } else {
+        button(text("  Parent  "))
+    };
+
+    let parent_btn = parent_btn
+        .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+            iced::widget::button::Style {
+                background: Some(iced::Background::Color(colors.bg_panel)),
+                text_color: colors.text_primary,
+                border: iced::Border::default().color(colors.border_low).width(1),
+                ..Default::default()
+            }
+        });
+
+    let dir_str = state.current_dir.to_string_lossy().to_string();
+    let total_files = state.images.len();
+    let selected_num = if total_files > 0 {
+        state.active_image_idx + 1
+    } else {
+        0
+    };
+    let stats_str = format!("{} Files | Selected: {} / {}", total_files, selected_num, total_files);
+
+    let toolbar = row![
+        parent_btn,
+        text("  ").size(8),
+        text(dir_str.clone())
+            .size(12)
+            .style(move |_theme| iced::widget::text::Style {
+                color: Some(colors.text_primary),
+            }),
+    ]
+    .align_y(iced::Alignment::Center)
+    .width(Length::Fill);
+
+    let stats = text(stats_str.clone())
+        .size(11)
+        .style(move |_theme| iced::widget::text::Style {
+            color: Some(colors.text_secondary),
+        });
+
+    let grid = thumbnail_grid::view(state, colors);
+
+    container(
+        column![
+            toolbar,
+            horizontal_rule(1),
+            stats,
+            horizontal_rule(1),
+            grid,
+        ]
+        .spacing(4)
+        .width(Length::Fill)
+        .height(Length::Fill),
+    )
+    .width(Length::Fill)
+    .height(Length::Fill)
+    .style(move |_theme| iced::widget::container::Style {
+        background: Some(iced::Background::Color(colors.bg_panel)),
+        border: iced::Border::default()
+            .color(colors.border_low)
+            .width(1)
+            .rounded(8),
+        ..Default::default()
+    })
+    .into()
+}
+
+/// Viewer mode — single image with navigation arrows and zoom.
+fn view_viewer(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    if state.images.is_empty() {
+        return column![
+            text("No images found in this folder.").style(move |_theme| iced::widget::text::Style {
+                color: Some(colors.text_secondary),
+            }),
+            button(text("Browse Mode")).on_press(Message::ViewerNavigate(
+                state.current_dir.clone(),
+            )),
+        ]
+        .align_x(iced::Alignment::Center)
+        .width(Length::Fill)
+        .height(Length::Fill)
+        .into();
+    }
+
+    let total = state.images.len();
+    let current_idx = state.active_image_idx;
+    let filename = state
+        .images
+        .get(current_idx)
+        .and_then(|p| p.file_name())
+        .map(|n| n.to_string_lossy().to_string())
+        .unwrap_or_default();
+
+    let edit_path = state.images[current_idx].clone();
+    let info_bar = row![
+        text(filename.clone()).style(move |_theme| iced::widget::text::Style {
+            color: Some(colors.text_primary),
+        }),
+        Space::with_width(Length::Fill),
+        text(format!("{} / {}", current_idx + 1, total)).style(move |_theme| iced::widget::text::Style {
+            color: Some(colors.text_secondary),
+        }),
+        text("  "),
+        button(text("Edit")).on_press(Message::ViewerOpenFile(edit_path))
+            .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+                iced::widget::button::Style {
+                    background: Some(iced::Background::Color(colors.accent)),
+                    text_color: iced::Color::WHITE,
+                    border: iced::Border::default().color(colors.accent).width(1),
+                    ..Default::default()
+                }
+            }),
+    ]
+    .align_y(iced::Alignment::Center)
+    .width(Length::Fill);
+
+    let image_area: Element<'static, Message> = if let Some((pixels, w, h)) = &state.preview_pixels {
+        let handle: iced::widget::image::Handle = iced::widget::image::Handle::from_rgba(
+            *w, *h, pixels.clone(),
+        );
+        let img = iced::widget::Image::new(handle)
+            .width(Length::Fill)
+            .height(Length::Fill);
+
+        let prev_path = state.images[current_idx].clone();
+        let prev_btn = button(text("  <  "))
+            .on_press(Message::ViewerNavigate(prev_path))
+            .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+                iced::widget::button::Style {
+                    background: Some(iced::Background::Color(iced::Color::from_rgba(
+                        0.0, 0.0, 0.0, 0.5,
+                    ))),
+                    text_color: iced::Color::WHITE,
+                    border: iced::Border::default().rounded(8),
+                    ..Default::default()
+                }
+            });
+
+        let next_idx = if current_idx + 1 < total {
+            current_idx + 1
+        } else {
+            0
+        };
+        let next_path = state.images[next_idx].clone();
+        let next_btn = button(text("  >  "))
+            .on_press(Message::ViewerNavigate(next_path))
+            .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+                iced::widget::button::Style {
+                    background: Some(iced::Background::Color(iced::Color::from_rgba(
+                        0.0, 0.0, 0.0, 0.5,
+                    ))),
+                    text_color: iced::Color::WHITE,
+                    border: iced::Border::default().rounded(8),
+                    ..Default::default()
+                }
+            });
+
+        row![prev_btn, img, next_btn]
+            .width(Length::Fill)
+            .height(Length::Fill)
+            .into()
+    } else if let Some(err) = &state.load_error {
+        column![text(format!("Failed to load: {}", err)).style(move |_theme| {
+            iced::widget::text::Style {
+                color: Some(colors.danger),
+            }
+        })]
+        .align_x(iced::Alignment::Center)
+        .width(Length::Fill)
+        .height(Length::Fill)
+        .into()
+    } else {
+        column![text("Loading...").style(move |_theme| iced::widget::text::Style {
+            color: Some(colors.text_secondary),
+        })]
+        .align_x(iced::Alignment::Center)
+        .width(Length::Fill)
+        .height(Length::Fill)
+        .into()
+    };
+
+    column![info_bar, horizontal_rule(1), image_area]
+        .width(Length::Fill)
+        .height(Length::Fill)
+        .into()
+}
diff --git a/hcie-iced-app/crates/hcie-iced-gui/src/viewer/thumbnail_grid.rs b/hcie-iced-app/crates/hcie-iced-gui/src/viewer/thumbnail_grid.rs
new file mode 100644
index 0000000..3f76e7e
--- /dev/null
+++ b/hcie-iced-app/crates/hcie-iced-gui/src/viewer/thumbnail_grid.rs
@@ -0,0 +1,159 @@
+//! Thumbnail grid panel with lazy loading.
+//!
+//! Displays a responsive grid of image thumbnails that load lazily
+//! (max 3 per frame to avoid stalls). Each card shows the thumbnail,
+//! filename, and selection highlight.
+
+use crate::app::Message;
+use crate::theme::ThemeColors;
+use crate::viewer::ViewerState;
+use iced::widget::{button, column, container, row, scrollable, text, Space};
+use iced::{Element, Length};
+
+/// Thumbnail card width in pixels.
+const CARD_W: f32 = 130.0;
+/// Thumbnail card spacing.
+const CARD_SPACING: f32 = 8.0;
+/// Max thumbnails to load per frame.
+const LAZY_LOAD_BUDGET: usize = 3;
+
+/// Scale a color's alpha by a factor.
+fn color_alpha(color: iced::Color, alpha: f32) -> iced::Color {
+    iced::Color {
+        a: color.a * alpha,
+        ..color
+    }
+}
+
+/// Build the thumbnail grid view.
+pub fn view(state: &ViewerState, colors: ThemeColors) -> Element<'static, Message> {
+    if state.images.is_empty() {
+        return container(
+            text("No images found in this folder.").style(move |_theme| {
+                iced::widget::text::Style {
+                    color: Some(colors.text_secondary),
+                }
+            }),
+        )
+        .width(Length::Fill)
+        .height(Length::Fill)
+        .center_x(Length::Fill)
+        .center_y(Length::Fill)
+        .into();
+    }
+
+    let avail_w = 600.0_f32;
+    let cols = ((avail_w / (CARD_W + CARD_SPACING)).floor().max(1.0)) as usize;
+
+    let mut grid_rows = column![].spacing(CARD_SPACING);
+    let total = state.images.len();
+    let mut index = 0;
+
+    while index < total {
+        let mut row_items = row![].spacing(CARD_SPACING);
+        for _ in 0..cols {
+            if index >= total {
+                break;
+            }
+            let card = thumbnail_card(state, index, colors);
+            row_items = row_items.push(card);
+            index += 1;
+        }
+        grid_rows = grid_rows.push(row_items);
+    }
+
+    scrollable(grid_rows.width(Length::Fill).padding(8))
+        .height(Length::Fill)
+        .into()
+}
+
+/// Render a single thumbnail card.
+fn thumbnail_card(state: &ViewerState, index: usize, colors: ThemeColors) -> Element<'static, Message> {
+    let img_path = &state.images[index];
+    let is_selected = state.active_image_idx == index;
+    let filename = img_path
+        .file_name()
+        .map(|n| n.to_string_lossy().to_string())
+        .unwrap_or_default();
+
+    let display_name = if filename.chars().count() > 15 {
+        format!("{}...", filename.chars().take(12).collect::<String>())
+    } else {
+        filename.clone()
+    };
+
+    let thumb_content: Element<'static, Message> = if let Some((pixels, w, h)) = state.thumbnail_cache.get(img_path) {
+        let handle: iced::widget::image::Handle = iced::widget::image::Handle::from_rgba(
+            *w, *h, pixels.clone(),
+        );
+        let img = iced::widget::Image::new(handle)
+            .width(110)
+            .height(100);
+        img.into()
+    } else if state.failed_thumbnails.contains(img_path) {
+        text("Error").size(10).into()
+    } else {
+        text("...").size(10).into()
+    };
+
+    let card_content = column![
+        Space::with_height(4),
+        thumb_content,
+        Space::with_height(6),
+        text(display_name.clone())
+            .size(10)
+            .style(move |_theme| iced::widget::text::Style {
+                color: Some(colors.text_primary),
+            }),
+    ]
+    .spacing(2)
+    .align_x(iced::Alignment::Center)
+    .width(CARD_W);
+
+    let frame_color = if is_selected {
+        color_alpha(colors.accent, 0.12)
+    } else {
+        colors.bg_app
+    };
+    let border_color = if is_selected {
+        colors.accent
+    } else {
+        colors.border_low
+    };
+
+    button(card_content)
+        .on_press(Message::ViewerSelectFile(img_path.clone()))
+        .width(CARD_W)
+        .style(move |_theme: &iced::Theme, _status: iced::widget::button::Status| {
+            iced::widget::button::Style {
+                background: Some(iced::Background::Color(frame_color)),
+                text_color: colors.text_primary,
+                border: iced::Border::default()
+                    .color(border_color)
+                    .width(if is_selected { 2.0 } else { 1.0 })
+                    .rounded(6),
+                ..Default::default()
+            }
+        })
+        .into()
+}
+
+/// Count how many thumbnails need loading.
+pub fn count_unloaded(state: &ViewerState) -> usize {
+    state
+        .images
+        .iter()
+        .filter(|p| !state.thumbnail_cache.contains_key(*p) && !state.failed_thumbnails.contains(*p))
+        .count()
+}
+
+/// Get the next batch of images to load thumbnails for.
+pub fn next_load_batch(state: &ViewerState) -> Vec<std::path::PathBuf> {
+    state
+        .images
+        .iter()
+        .filter(|p| !state.thumbnail_cache.contains_key(*p) && !state.failed_thumbnails.contains(*p))
+        .take(LAZY_LOAD_BUDGET)
+        .cloned()
+        .collect()
+}