fix(editor): ensure canvas is cleared when background color is invalid to prevent ghosting (#11458)

Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

packages/excalidraw/renderer/helpers.test.ts

@@ -0,0 +1,90 @@
+import { COLOR_WHITE } from "@excalidraw/common";
+
+import { bootstrapCanvas } from "./helpers";
+
+const setup = () => {
+  const canvas = document.createElement("canvas");
+  canvas.width = 200;
+  canvas.height = 100;
+  const context = canvas.getContext("2d")!;
+  const clearRect = vi.spyOn(context, "clearRect");
+  const fillRect = vi.spyOn(context, "fillRect");
+  return { canvas, context, clearRect, fillRect };
+};
+
+const run = (viewBackgroundColor: unknown) => {
+  const { canvas, context, clearRect, fillRect } = setup();
+  bootstrapCanvas({
+    canvas,
+    scale: 1,
+    normalizedWidth: 200,
+    normalizedHeight: 100,
+    viewBackgroundColor: viewBackgroundColor as string,
+  });
+  return { context, clearRect, fillRect };
+};
+
+describe("bootstrapCanvas background painting", () => {
+  it("skips clearRect for an opaque hex color (fill fully repaints)", () => {
+    const { clearRect, fillRect } = run("#ffffff");
+    expect(clearRect).not.toHaveBeenCalled();
+    expect(fillRect).toHaveBeenCalledTimes(1);
+  });
+
+  it("skips clearRect for a 3-digit opaque hex color", () => {
+    const { clearRect, fillRect } = run("#fff");
+    expect(clearRect).not.toHaveBeenCalled();
+    expect(fillRect).toHaveBeenCalledTimes(1);
+  });
+
+  it("clears for a hex color with alpha (#RGBA / #RRGGBBAA)", () => {
+    expect(run("#ffff").clearRect).toHaveBeenCalledTimes(1);
+    expect(run("#ffffff80").clearRect).toHaveBeenCalledTimes(1);
+  });
+
+  it("clears and skips fill for the transparent keyword", () => {
+    const { clearRect, fillRect } = run("transparent");
+    expect(clearRect).toHaveBeenCalledTimes(1);
+    expect(fillRect).not.toHaveBeenCalled();
+  });
+
+  it("clears for rgba()/hsla() colors and still fills", () => {
+    const rgba = run("rgba(255, 0, 0, 0.5)");
+    expect(rgba.clearRect).toHaveBeenCalledTimes(1);
+    expect(rgba.fillRect).toHaveBeenCalledTimes(1);
+  });
+
+  // the ghosting bug (#10931): a corrupted value must never leave the prior
+  // frame on screen — we always clear when we can't prove the color is opaque
+  it("clears for a corrupted color value to prevent ghosting", () => {
+    expect(run("0000").clearRect).toHaveBeenCalledTimes(1);
+    expect(run("asdfgh").clearRect).toHaveBeenCalledTimes(1);
+  });
+
+  it("falls back to white when the color is rejected by the canvas", () => {
+    const { canvas, context } = setup();
+    // simulate a stale fillStyle left over from a previous frame's drawing
+    context.fillStyle = "#ff0000";
+    let fillStyleAtFillTime = "";
+    vi.spyOn(context, "fillRect").mockImplementation(() => {
+      fillStyleAtFillTime = context.fillStyle as string;
+    });
+
+    bootstrapCanvas({
+      canvas,
+      scale: 1,
+      normalizedWidth: 200,
+      normalizedHeight: 100,
+      viewBackgroundColor: "not-a-color",
+    });
+
+    // not the stale red — the seeded default
+    expect(fillStyleAtFillTime).toBe(COLOR_WHITE);
+  });
+
+  it("clears for a non-string background", () => {
+    const { clearRect, fillRect } = run(undefined);
+    expect(clearRect).toHaveBeenCalledTimes(1);
+    expect(fillRect).not.toHaveBeenCalled();
+  });
+});

packages/excalidraw/renderer/helpers.ts

@@ -1,4 +1,4 @@
-import { THEME, applyDarkModeFilter } from "@excalidraw/common";
+import { COLOR_WHITE, THEME, applyDarkModeFilter } from "@excalidraw/common";
 
 import type { StaticCanvasRenderConfig } from "../scene/types";
 import type { AppState, StaticCanvasAppState } from "../types";
@@ -53,21 +53,31 @@ export const bootstrapCanvas = ({
 
   // Paint background
   if (typeof viewBackgroundColor === "string") {
-    const hasTransparence =
-      viewBackgroundColor === "transparent" ||
-      viewBackgroundColor.length === 5 || // #RGBA
-      viewBackgroundColor.length === 9 || // #RRGGBBA
-      /(hsla|rgba)\(/.test(viewBackgroundColor);
-    if (hasTransparence) {
+    // An opaque fill repaints every pixel, so clearRect would be redundant.
+    // For anything else — transparency, or a value we can't be certain about
+    // (e.g. corrupted persisted state like "0000") — clear first so the
+    // previous frame can't bleed through.
+    //
+    // We skip opaque #RRGGBB and #RGB hex colors as a quick optimization.
+    const isOpaque = /^#([0-9a-f]{3}|[0-9a-f]{6})$/i.test(viewBackgroundColor);
+
+    if (!isOpaque) {
       context.clearRect(0, 0, normalizedWidth, normalizedHeight);
     }
-    context.save();
-    context.fillStyle = applyDarkModeFilter(
-      viewBackgroundColor,
-      theme === THEME.DARK,
-    );
-    context.fillRect(0, 0, normalizedWidth, normalizedHeight);
-    context.restore();
+
+    if (viewBackgroundColor !== "transparent") {
+      context.save();
+      // The canvas silently ignores an invalid fillStyle, which would leave a
+      // stale color from a previous draw. Seed a sane default so corrupted
+      // values fall back to white instead of painting garbage.
+      context.fillStyle = COLOR_WHITE;
+      context.fillStyle = applyDarkModeFilter(
+        viewBackgroundColor,
+        theme === THEME.DARK,
+      );
+      context.fillRect(0, 0, normalizedWidth, normalizedHeight);
+      context.restore();
+    }
   } else {
     context.clearRect(0, 0, normalizedWidth, normalizedHeight);
   }

packages/excalidraw/renderer/staticScene.ts

@@ -1,5 +1,6 @@
 import {
   applyDarkModeFilter,
+  COLOR_WHITE,
   FRAME_STYLE,
   THEME,
   throttleRAF,
@@ -204,7 +205,13 @@ const renderLinkIcon = (
         window.devicePixelRatio * appState.zoom.value,
         window.devicePixelRatio * appState.zoom.value,
       );
-      linkCanvasCacheContext.fillStyle = appState.viewBackgroundColor || "#fff";
+
+      // Seed a sane default so a corrupted color (silently rejected by the
+      // canvas) falls back to white instead of a stale fillStyle.
+      linkCanvasCacheContext.fillStyle = COLOR_WHITE;
+      linkCanvasCacheContext.fillStyle =
+        appState.viewBackgroundColor || COLOR_WHITE;
+
       linkCanvasCacheContext.fillRect(0, 0, width, height);
 
       if (canvasKey === "elementLink") {