diff --git a/render-wasm/src/render.rs b/render-wasm/src/render.rs
index 8aa067dae2..050f8820f4 100644
--- a/render-wasm/src/render.rs
+++ b/render-wasm/src/render.rs
@@ -1052,10 +1052,16 @@ impl RenderState {
     }
 
     pub fn apply_render_to_final_canvas(&mut self) -> Result<()> {
+        let current_tile = *self
+            .current_tile
+            .as_ref()
+            .ok_or(Error::CriticalError("Current tile not found".to_string()))?;
+
         // During interactive transforms we render tiles directly into Target; updating the cache
         // (snapshot -> atlas blit -> tiles.add) can force GPU stalls. Defer cache rebuild until
         // the interaction ends.
         if self.options.is_interactive_transform() {
+            self.surfaces.paint_debug_tile_overlay(&current_tile);
             let tile_rect = self.get_current_aligned_tile_bounds()?;
             self.surfaces.draw_current_tile_into_backbuffer(
                 &tile_rect,
@@ -1068,6 +1074,7 @@ impl RenderState {
         // Viewer masked passes render a partial scene. Reusing the tile texture cache would
         // SrcOver-blend onto textures from the previous pass and leak pixels into the blob.
         if self.viewer_masked_pass() {
+            self.surfaces.paint_debug_tile_overlay(&current_tile);
             // Use viewbox-aligned bounds (not grid-snapped) to match interactive-transform
             // compositing and avoid a visible offset vs the DOM canvas.
             let tile_rect = self.get_current_tile_bounds()?;
@@ -1091,11 +1098,6 @@ impl RenderState {
         // positions would be wrong — only save to the tile HashMap.
         let tile_rect = self.get_current_aligned_tile_bounds()?;
 
-        let current_tile = *self
-            .current_tile
-            .as_ref()
-            .ok_or(Error::CriticalError("Current tile not found".to_string()))?;
-
         if self.tile_atlas_flushed {
             crate::get_gpu_state().context.flush_and_submit();
         }
@@ -3845,6 +3847,7 @@ impl RenderState {
                         if self.options.is_interactive_transform() {
                             // During drag, avoid snapshot-based caching. Draw Current directly
                             // into Target (and Cache) to reduce stalls.
+                            self.surfaces.paint_debug_tile_overlay(&current_tile);
                             self.surfaces.draw_current_tile_into_backbuffer(
                                 &tile_rect,
                                 self.background_color,
diff --git a/render-wasm/src/render/surfaces.rs b/render-wasm/src/render/surfaces.rs
index b8f4b51c9d..b93cd9628e 100644
--- a/render-wasm/src/render/surfaces.rs
+++ b/render-wasm/src/render/surfaces.rs
@@ -1204,6 +1204,33 @@ impl Surfaces {
         canvas.restore();
     }
 
+    /// Debug: semi-transparent tint unique per tile coords, baked into Current
+    /// before atlas/backbuffer blit so tile boundaries are visible on screen.
+    pub fn paint_debug_tile_overlay(&mut self, tile: &Tile) {
+        let canvas = self.current.canvas();
+        let rect = skia::Rect::from(TILE_DRAWABLE_RECT);
+
+        // Stable pseudo-random RGB from tile coords (same tile → same color).
+        let h = (tile.x() as u32)
+            .wrapping_mul(374761393)
+            .wrapping_add((tile.y() as u32).wrapping_mul(668265263))
+            .wrapping_add(0x9E37_79B9);
+        let r = ((h >> 0) & 0xFF) as u8;
+        let g = ((h >> 8) & 0xFF) as u8;
+        let b = ((h >> 16) & 0xFF) as u8;
+
+        let mut paint = skia::Paint::default();
+        paint.set_anti_alias(true);
+        paint.set_style(skia::PaintStyle::Fill);
+        paint.set_color(skia::Color::from_argb(96, r, g, b));
+        canvas.draw_rect(rect, &paint);
+
+        paint.set_style(skia::PaintStyle::Stroke);
+        paint.set_stroke_width(2.0);
+        paint.set_color(skia::Color::from_argb(220, r, g, b));
+        canvas.draw_rect(rect, &paint);
+    }
+
     pub fn draw_current_tile_into_tile_atlas(
         &mut self,
         tile_viewbox: &TileViewbox,
@@ -1212,6 +1239,8 @@ impl Surfaces {
         skip_cache_surface: bool,
         tile_doc_rect: skia::Rect,
     ) {
+        self.paint_debug_tile_overlay(tile);
+
         let gpu_state = get_gpu_state();
         let src = skia::Rect::from(TILE_DRAWABLE_RECT);
         let sampling = self.sampling_options;