perf: eliminate crosshair input lag by using immediate cursor position and forced view redraws

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

@@ -691,6 +691,11 @@ pub enum Message {
         x: u32,
         y: u32,
     },
+    /// Lightweight no-op message emitted by the canvas overlay on every
+    /// `CursorMoved` event.  Its sole purpose is to trigger a `view()`
+    /// rebuild so the crosshair overlay redraws at the latest cursor
+    /// position during idle (non-drawing) pointer movement.
+    CursorOverlayRedraw,
     ModifiersChanged(iced::keyboard::Modifiers),
     SpacePanChanged(bool),
 
@@ -3554,6 +3559,11 @@ impl HcieIcedApp {
                 self.canvas_cursor_pos = Some((x, y));
             }
 
+            // No-op — this message exists solely to trigger a view() rebuild
+            // so the crosshair overlay redraws at the latest cursor position.
+            // See Strategy 1 in the cursor lag fix.
+            Message::CursorOverlayRedraw => {}
+
             Message::CanvasZoomSet(z) => {
                 let doc = &mut self.documents[self.active_doc];
                 doc.zoom = z.clamp(ZOOM_MIN, ZOOM_MAX);

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

@@ -1169,7 +1169,7 @@ impl canvas::Program<Message> for OverlayProgram {
         renderer: &iced::Renderer,
         _theme: &iced::Theme,
         bounds: Rectangle,
-        _cursor: Cursor,
+        cursor: Cursor,
     ) -> Vec<canvas::Geometry> {
         let (origin_x, origin_y, _display_w, _display_h) = self.canvas_origin(bounds.size());
         let mut geometries = Vec::new();
@@ -2010,8 +2010,12 @@ impl canvas::Program<Message> for OverlayProgram {
         }
 
         // ── Crosshair cursor ─────────────────────────────────────────────
-        if state.is_hovered {
+        // Strategy 4: Use the `cursor` parameter from iced's current render
-            if let Some(cursor) = state.cursor_pos {
+        // frame instead of the retained `state.cursor_pos` (which lags by
+        // one Elm update cycle).  This eliminates the 1-frame positional
+        // delay between the OS pointer and the brush-circle indicator.
+        if cursor.is_over(bounds) {
+            if let Some(cursor_point) = cursor.position_in(bounds) {
                 let mut crosshair_frame = Frame::new(renderer, bounds.size());
                 let crosshair_size = 10.0;
                 let crosshair_color = iced::Color::from_rgb(1.0, 1.0, 1.0);
@@ -2023,7 +2027,7 @@ impl canvas::Program<Message> for OverlayProgram {
                         | hcie_engine_api::Tool::Spray
                 ) {
                     let footprint =
-                        Path::circle(cursor, (self.brush_size * self.zoom * 0.5).max(1.0));
+                        Path::circle(cursor_point, (self.brush_size * self.zoom * 0.5).max(1.0));
                     crosshair_frame.stroke(
                         &footprint,
                         Stroke {
@@ -2035,8 +2039,8 @@ impl canvas::Program<Message> for OverlayProgram {
                 }
                 crosshair_frame.stroke(
                     &Path::line(
-                        Point::new(cursor.x - crosshair_size, cursor.y),
+                        Point::new(cursor_point.x - crosshair_size, cursor_point.y),
-                        Point::new(cursor.x + crosshair_size, cursor.y),
+                        Point::new(cursor_point.x + crosshair_size, cursor_point.y),
                     ),
                     Stroke {
                         style: canvas::stroke::Style::Solid(crosshair_color),
@@ -2046,8 +2050,8 @@ impl canvas::Program<Message> for OverlayProgram {
                 );
                 crosshair_frame.stroke(
                     &Path::line(
-                        Point::new(cursor.x, cursor.y - crosshair_size),
+                        Point::new(cursor_point.x, cursor_point.y - crosshair_size),
-                        Point::new(cursor.x, cursor.y + crosshair_size),
+                        Point::new(cursor_point.x, cursor_point.y + crosshair_size),
                     ),
                     Stroke {
                         style: canvas::stroke::Style::Solid(crosshair_color),
@@ -2147,6 +2151,14 @@ impl canvas::Program<Message> for OverlayProgram {
                 }
             }
         }
+        // Strategy 1: Emit a lightweight redraw message on every CursorMoved
+        // event so iced triggers a view() rebuild and the overlay's draw()
+        // runs with the latest cursor position.  Without this, iced may
+        // skip repaints during idle cursor movement because no application
+        // state changed.
+        if matches!(event, canvas::Event::Mouse(mouse::Event::CursorMoved { .. })) {
+            return (canvas::event::Status::Ignored, Some(Message::CursorOverlayRedraw));
+        }
         // Always pass events through to the shader widget below
         (canvas::event::Status::Ignored, None)
     }