feat: add local pan offset to eliminate one-frame panning latency in shader canvas

hcie-iced-app/crates/hcie-iced-gui/src/canvas/shader_canvas.rs

@@ -893,6 +893,8 @@ pub struct CanvasShaderState {
     pub middle_pressed: bool,
     /// Distinguishes Space+left temporary pan from a physical middle-button pan.
     pub space_left_pan: bool,
+    /// Immediate local pan offset maintained during active panning to eliminate 1-frame Elm update latency.
+    pub local_pan_offset: Option<Vector>,
 }
 
 /// Canvas shader program — implements `iced::widget::shader::Program`.
@@ -988,6 +990,10 @@ impl shader::Program<Message> for CanvasShaderProgram {
         cursor: mouse::Cursor,
         _shell: &mut Shell<'_, Message>,
     ) -> (iced::event::Status, Option<Message>) {
+        if !state.middle_pressed {
+            state.local_pan_offset = None;
+        }
+
         match event {
             shader::Event::Mouse(mouse_event) => {
                 match mouse_event {
@@ -1005,7 +1011,7 @@ impl shader::Program<Message> for CanvasShaderProgram {
                             self.engine_w as f32,
                             self.engine_h as f32,
                             self.zoom,
-                            self.pan_offset,
+                            state.local_pan_offset.unwrap_or(self.pan_offset),
                         );
 
                         // Left mouse drag (drawing)
@@ -1024,7 +1030,8 @@ impl shader::Program<Message> for CanvasShaderProgram {
                             let start = state.pan_start.unwrap();
                             let delta = position - start;
                             state.pan_start = Some(position);
-                            let mut new_pan = self.pan_offset;
+                            let current_pan = state.local_pan_offset.unwrap_or(self.pan_offset);
+                            let mut new_pan = current_pan;
                             new_pan.x += delta.x;
                             new_pan.y += delta.y;
 
@@ -1048,6 +1055,8 @@ impl shader::Program<Message> for CanvasShaderProgram {
                                 (bounds.height - min_vis_h).max(0.0),
                             ) - default_oy;
 
+                            state.local_pan_offset = Some(new_pan);
+
                             return (
                                 iced::event::Status::Captured,
                                 Some(Message::CanvasPanZoom {
@@ -1079,6 +1088,7 @@ impl shader::Program<Message> for CanvasShaderProgram {
                                 state.middle_pressed = true;
                                 state.space_left_pan = true;
                                 state.pan_start = cursor.position();
+                                state.local_pan_offset = Some(self.pan_offset);
                                 return (iced::event::Status::Captured, None);
                             }
                             let local_pos = {
@@ -1105,6 +1115,7 @@ impl shader::Program<Message> for CanvasShaderProgram {
                             if let Some(pos) = cursor.position() {
                                 state.middle_pressed = true;
                                 state.pan_start = Some(pos);
+                                state.local_pan_offset = Some(self.pan_offset);
                                 return (iced::event::Status::Captured, None);
                             }
                         } else if button == mouse::Button::Right {
@@ -1142,6 +1153,7 @@ impl shader::Program<Message> for CanvasShaderProgram {
                                 state.middle_pressed = false;
                                 state.space_left_pan = false;
                                 state.pan_start = None;
+                                state.local_pan_offset = None;
                                 return (iced::event::Status::Captured, None);
                             }
                             state.left_pressed = false;
@@ -1152,6 +1164,7 @@ impl shader::Program<Message> for CanvasShaderProgram {
                         } else if button == mouse::Button::Middle {
                             state.middle_pressed = false;
                             state.pan_start = None;
+                            state.local_pan_offset = None;
                             return (iced::event::Status::Captured, None);
                         }
                     }
@@ -1240,7 +1253,7 @@ impl shader::Program<Message> for CanvasShaderProgram {
     /// and pixel data so `prepare()` can upload only the changed sub-region.
     fn draw(
         &self,
-        _state: &Self::State,
+        state: &Self::State,
         _cursor: mouse::Cursor,
         bounds: Rectangle,
     ) -> Self::Primitive {