🐛 Fix playground rendering at non-unit device pixel ratio

The playground called _init and _set_render_options with the DPR,
which sets the ratio but leaves the Skia target, backbuffer and
UI surfaces at CSS resolution. The canvas drawing buffer is
physical (CSS x DPR), so at DPR > 1 the smaller surface anchored
to the bottom-left of the framebuffer and left the top and right
edges unpainted.

Call _resize_viewbox after setting the DPR, the same sequence the
application uses in resize-canvas!, so the renderer surfaces
match the physical drawing buffer.

Cover DPR 1, 1.25 and 2 in the adapter test.

AI-assisted-by: deepseek-flash

frontend/resources/wasm-playground/js/lib.js

@@ -37,13 +37,14 @@ export function createRendererAdapter(module, scheduler = globalThis) {
     handle = module.GL.registerContext(context, { majorVersion: 2 });
     module.GL.makeContextCurrent(handle);
     context.getExtension("WEBGL_debug_renderer_info");
-    module._init(
-      Math.round(canvas.width / dpr),
-      Math.round(canvas.height / dpr),
-    );
+    const width = Math.round(canvas.width / dpr);
+    const height = Math.round(canvas.height / dpr);
+    module._init(width, height);
     module._set_render_options(0, dpr);
     // RawBrowser::Chrome = 1; use the same default browser as the benchmark.
     module._set_browser?.(1);
+    // Setting DPR does not resize the renderer's surfaces or framebuffer.
+    module._resize_viewbox(width, height);
   }
 
   function renderFull(flags = 4) {

frontend/scripts/renderer-benchmarks/tests/adapter.test.js

@@ -47,6 +47,7 @@ function fixture() {
     },
     _init() {},
     _set_render_options() {},
+    _resize_viewbox() {},
     _set_browser() {},
     _set_view_start() {
       calls.start++;
@@ -74,6 +75,41 @@ function fixture() {
   return { adapter, state, calls, canvas, frames, timers, module };
 }
 
+for (const dpr of [1, 1.25, 2]) {
+  test(`adapter initializes the full drawing buffer at DPR ${dpr}`, () => {
+    const size = { width: 0, height: 0 };
+    let rendererDpr = 1;
+    const module = {
+      GL: {
+        registerContext: () => 1,
+        makeContextCurrent() {},
+      },
+      _init(width, height) {
+        Object.assign(size, { width, height });
+      },
+      _set_render_options(_flags, value) {
+        rendererDpr = value;
+      },
+      _resize_viewbox(width, height) {
+        // The renderer applies DPR when resizing, not when setting options.
+        Object.assign(size, {
+          width: width * rendererDpr,
+          height: height * rendererDpr,
+        });
+      },
+    };
+    const canvas = {
+      width: 1920 * dpr,
+      height: 1080 * dpr,
+      getContext: () => ({ getExtension() {} }),
+    };
+
+    createRendererAdapter(module).assignCanvas(canvas, dpr);
+
+    assert.deepEqual(size, { width: canvas.width, height: canvas.height });
+  });
+}
+
 test("wheel zoom flushes once per animation frame", () => {
   const f = fixture();
   const wheel = (deltaY, offsetX, offsetY) =>