fix type declarations for getContext on raw desktop backend

cli/tsc/dts/lib.deno.unstable.d.ts

@@ -231,6 +231,11 @@ declare namespace Deno {
       },
     );
 
+    getContext(
+      contextId: "bitmaprenderer",
+      options?: any,
+    ): ImageBitmapRenderingContext | null;
+    getContext(contextId: "webgpu", options?: any): GPUCanvasContext | null;
     getContext(
       contextId: OffscreenRenderingContextId,
       options?: any,

cli/tsc/dts/lib.deno_canvas.d.ts

@@ -252,6 +252,7 @@ interface GPUCanvasContext {
  * @category Canvas */
 declare var GPUCanvasContext: {
   prototype: GPUCanvasContext;
+  new (): GPUCanvasContext;
 };
 
 /** A rendering context that displays the contents of an {@linkcode ImageBitmap}
@@ -275,6 +276,7 @@ interface ImageBitmapRenderingContext {
  * @category Canvas */
 declare var ImageBitmapRenderingContext: {
   prototype: ImageBitmapRenderingContext;
+  new (): ImageBitmapRenderingContext;
 };
 
 /** A canvas that can be rendered to off the main thread and without being

tests/specs/check/unsafe_window_surface_getcontext/__test__.jsonc

@@ -0,0 +1,5 @@
+{
+  "args": "check main.ts",
+  "output": "main.out",
+  "exitCode": 0
+}

tests/specs/check/unsafe_window_surface_getcontext/main.out

@@ -0,0 +1 @@
+Check [WILDCARD]main.ts

tests/specs/check/unsafe_window_surface_getcontext/main.ts

@@ -0,0 +1,22 @@
+// Type-level checks that Deno.UnsafeWindowSurface.getContext narrows per
+// context id, mirroring the OffscreenCanvas.getContext overloads.
+
+function assertType<T>(_value: T): void {}
+
+declare const surface: Deno.UnsafeWindowSurface;
+
+assertType<GPUCanvasContext | null>(surface.getContext("webgpu"));
+assertType<ImageBitmapRenderingContext | null>(
+  surface.getContext("bitmaprenderer"),
+);
+declare const contextId: OffscreenRenderingContextId;
+assertType<OffscreenRenderingContext | null>(surface.getContext(contextId));
+
+// The constructor objects have construct signatures, so instanceof narrows.
+declare const context: unknown;
+if (context instanceof GPUCanvasContext) {
+  assertType<GPUCanvasContext>(context);
+}
+if (context instanceof ImageBitmapRenderingContext) {
+  assertType<ImageBitmapRenderingContext>(context);
+}