🐛 Fix drag and drop cache eligibility rules

render-wasm/src/render.rs

@@ -391,6 +391,9 @@ pub(crate) struct RenderState {
 pub struct InteractiveDragCrop {
     pub src_doc_bounds: Rect,
     pub src_selrect: Rect,
+    /// True if the captured crop bounds were fully inside the viewport at capture time.
+    /// Used to avoid serving partial/offscreen crops during interactive drag.
+    pub fits_viewport_at_capture: bool,
     pub image: skia::Image,
 }
 
@@ -444,13 +447,30 @@ impl RenderState {
             let Some(m) = tree.get_modifier(&node_id) else {
                 return false;
             };
-            return crate::math::is_move_only_matrix(m);
+            // Only allow using the cached pixels for pure translations.
+            // For non-translation transforms (scale/rotate/skew), cached pixels won't match.
+            if !crate::math::is_move_only_matrix(m) {
+                return false;
+            }
+
+            let Some(crop) = self.backbuffer_crop_cache.get(&node_id) else {
+                return false;
+            };
+            if !crop.fits_viewport_at_capture {
+                return false;
+            }
+
+            // Additionally require this node to be safe to serve from a rectangular backbuffer
+            // crop while moving; otherwise it must be rendered live (e.g. text, overflow frames).
+            return tree
+                .get(&node_id)
+                .is_some_and(|s| s.is_safe_for_drag_crop_cache(tree));
         }
 
-        // Invalidate cached top-level pixels whenever the moving content overlaps
-        // the cached pixel area. Use `src_doc_bounds` because it's the exact bounds
-        // captured from the Backbuffer crop (more reliable than extents derived
-        // from layout/layout-less container heuristics).
+        // If the moving content overlaps this cached crop, do not use the cached pixels
+        // for this frame. We intentionally keep the cache entry: overlap is typically
+        // transient during drag, and once the moving content leaves the area the crop
+        // becomes valid again (stationary shape unchanged).
         if let Some(moved) = moved_bounds {
             let intersects = self
                 .backbuffer_crop_cache
@@ -458,25 +478,12 @@ impl RenderState {
                 .is_some_and(|crop| moved.intersects(crop.src_doc_bounds));
 
             if intersects {
-                // Simplest "automatic invalidation": once something moves over this cached
-                // area, drop the cached crop so it won't be reused again until the next
-                // full-frame rebuild.
-                self.backbuffer_crop_cache.remove(&node_id);
                 return false;
             }
         }
         true
     }
 
-    fn is_recortable_for_drag_crop(&self, shape: &Shape) -> bool {
-        // "Recortable" (happy path): the shape is fully represented by the pixels
-        // already in Backbuffer and can be moved as a texture during drag.
-        shape.blur.is_none()
-            && shape.shadows.is_empty()
-            && (shape.opacity - 1.0).abs() <= 1e-4
-            && shape.blend_mode().0 == skia::BlendMode::SrcOver
-    }
-
     pub fn try_new(width: i32, height: i32) -> Result<RenderState> {
         // This needs to be done once per WebGL context.
         let mut gpu_state = GpuState::try_new()?;
@@ -1649,9 +1656,6 @@ impl RenderState {
             if shape.hidden {
                 continue;
             }
-            if !self.is_recortable_for_drag_crop(shape) {
-                continue;
-            }
 
             let doc_bounds = self.get_cached_extrect(shape, tree, 1.0);
             if !doc_bounds.intersects(viewport) {
@@ -1707,11 +1711,16 @@ impl RenderState {
                 src_irect.right as f32 / scale + vb_left,
                 src_irect.bottom as f32 / scale + vb_top,
             );
+            let fits_viewport_at_capture = doc_bounds.left >= viewport.left
+                && doc_bounds.top >= viewport.top
+                && doc_bounds.right <= viewport.right
+                && doc_bounds.bottom <= viewport.bottom;
             self.backbuffer_crop_cache.insert(
                 id,
                 InteractiveDragCrop {
                     src_doc_bounds,
                     src_selrect: selrect,
+                    fits_viewport_at_capture,
                     image,
                 },
             );
@@ -3024,16 +3033,31 @@ impl RenderState {
                             dst_doc_rect.height() * scale,
                         );
 
-                        // let canvas = self.surfaces.canvas_and_mark_dirty(target_surface);
                         let canvas = self.surfaces.canvas(target_surface);
                         canvas.save();
                         canvas.reset_matrix();
+                        // If the crop includes shadows/blur (extrect pixels outside the fill/stroke
+                        // silhouette), do NOT apply the silhouette clip or we'd cut those pixels.
+                        let should_clip_crop = element.shadows.is_empty() && element.blur.is_none();
+                        if should_clip_crop {
+                            if let Some(clip_path) = element.drag_crop_clip_path() {
+                                let mut doc_to_tile = Matrix::new_identity();
+                                // Map document-space coordinates into tile pixels.
+                                // Rendering surfaces apply: scale(scale) then translate(translation) in doc units.
+                                // Equivalent point mapping: (doc + translation) * scale.
+                                doc_to_tile.post_translate((translation.0, translation.1));
+                                doc_to_tile.post_scale((scale, scale), None);
+                                let clip_path = clip_path.make_transform(&doc_to_tile);
+                                canvas.clip_path(&clip_path, skia::ClipOp::Intersect, true);
+                            }
+                        }
                         canvas.draw_image_rect(
                             crop_image,
                             None,
                             dst_tile_rect,
                             &skia::Paint::default(),
                         );
+
                         canvas.restore();
                     }
                     continue;

render-wasm/src/shapes.rs

@@ -1358,6 +1358,112 @@ impl Shape {
         }
     }
 
+    /// Same `concat` applied around [`center`](Self::center) as in `render_shape` (non-text branch).
+    fn shape_document_transform(&self) -> Matrix {
+        let c = self.center();
+        let mut m = self.transform;
+        m.post_translate(c);
+        m.pre_translate(-c);
+        m
+    }
+
+    /// Fill silhouette only, document space (matches fill rendering).
+    fn drag_crop_fill_clip_path_skia(&self) -> Option<skia::Path> {
+        match &self.shape_type {
+            Type::Rect(r) => {
+                let p = Path::new(shape_to_path::rect_segments(self, r.corners));
+                Some(p.to_skia_path(self.svg_attrs.as_ref()))
+            }
+            Type::Circle => {
+                let p = Path::new(shape_to_path::circle_segments(self));
+                Some(p.to_skia_path(self.svg_attrs.as_ref()))
+            }
+            Type::Path(_) | Type::Bool(_) => {
+                let sk = self.get_skia_path()?;
+                Some(sk.make_transform(&self.shape_document_transform()))
+            }
+            _ => None,
+        }
+    }
+
+    /// Whether this shape may use the backbuffer crop fast path during interactive drag.
+    ///
+    /// Conservative: only effects and fills that match what we snapshot and clip in
+    /// [`drag_crop_clip_path`](Self::drag_crop_clip_path). Text is never safe (glyph layout,
+    /// no `drag_crop_clip_path`).
+    pub fn is_safe_for_drag_crop_cache(&self, shapes_pool: ShapesPoolRef) -> bool {
+        if matches!(self.shape_type, Type::Text(_)) {
+            return false;
+        }
+
+        // If a frame shows overflow (clip_content=false) and its visible content exceeds the
+        // frame bounds, a cached crop anchored to the frame can easily become incorrect while
+        // moving (children can extend beyond selrect). Be conservative and render live.
+        if matches!(self.shape_type, Type::Frame(_)) && !self.clip_content {
+            let extrect = self.extrect(shapes_pool, 1.0);
+            let sr = self.selrect;
+            let exceeds = extrect.left < sr.left
+                || extrect.top < sr.top
+                || extrect.right > sr.right
+                || extrect.bottom > sr.bottom;
+            if exceeds {
+                return false;
+            }
+        }
+
+        let has_opaque_fill = self
+            .fills
+            .iter()
+            .any(|f| math::is_close_to(f.opacity(), 1.0));
+
+        self.blur.is_none()
+            && self.shadows.is_empty()
+            && (self.opacity - 1.0).abs() <= 1e-4
+            && self.blend_mode().0 == skia::BlendMode::SrcOver
+            && has_opaque_fill
+    }
+
+    /// Fill + visible strokes in **document space** for clipping interactive drag textures.
+    ///
+    /// The backbuffer crop uses an axis-aligned `extrect`; we clip the blit so backdrop pixels
+    /// outside the real silhouette (fill and stroke regions) are not smeared. Strokes use
+    /// [`stroke_to_path`](stroke_to_path) like the main renderer, then union with the fill path.
+    pub fn drag_crop_clip_path(&self) -> Option<skia::Path> {
+        let mut acc = self.drag_crop_fill_clip_path_skia()?;
+        if !self.has_visible_strokes() {
+            return Some(acc);
+        }
+
+        let shape_path = match &self.shape_type {
+            Type::Rect(r) => Path::new(shape_to_path::rect_segments(self, r.corners)),
+            Type::Circle => Path::new(shape_to_path::circle_segments(self)),
+            Type::Path(_) | Type::Bool(_) => self.shape_type.path()?.clone(),
+            _ => return Some(acc),
+        };
+
+        let path_transform = self.to_path_transform();
+        let apply_doc_transform = path_transform.is_some();
+
+        for stroke in self.visible_strokes() {
+            let Some(stroke_region) = stroke_to_path(
+                stroke,
+                &shape_path,
+                path_transform.as_ref(),
+                &self.selrect,
+                self.svg_attrs.as_ref(),
+            ) else {
+                continue;
+            };
+            let mut sk = stroke_region.to_skia_path(self.svg_attrs.as_ref());
+            if apply_doc_transform {
+                sk = sk.make_transform(&self.shape_document_transform());
+            }
+            acc = acc.op(&sk, skia::PathOp::Union).unwrap_or(acc);
+        }
+
+        Some(acc)
+    }
+
     fn transform_selrect(&mut self, transform: &Matrix) {
         if math::is_move_only_matrix(transform) {
             let tx = transform.translate_x();