Bug 1765816 - Expose front buffer canvas contents when painting for Firefox Screenshots for WebGPU. r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D144868

dom/webgpu/CanvasContext.cpp

@@ -10,6 +10,7 @@
 #include "nsDisplayList.h"
 #include "mozilla/dom/HTMLCanvasElement.h"
 #include "mozilla/gfx/CanvasManagerChild.h"
+#include "mozilla/layers/CanvasRenderer.h"
 #include "mozilla/layers/CompositableInProcessManager.h"
 #include "mozilla/layers/ImageDataSerializer.h"
 #include "mozilla/layers/LayersSurfaces.h"
@@ -126,6 +127,24 @@ void CanvasContext::SwapChainPresent() {
   }
 }
 
+bool CanvasContext::InitializeCanvasRenderer(
+    nsDisplayListBuilder* aBuilder, layers::CanvasRenderer* aRenderer) {
+  // This path is only used for rendering when we use the fallback Paint path,
+  // used by reftest-snapshot, printing and Firefox Screenshot.
+  if (!mHandle) {
+    return false;
+  }
+
+  layers::CanvasRendererData data;
+  data.mContext = this;
+  data.mSize = mGfxSize;
+  data.mIsOpaque = false;
+
+  aRenderer->Initialize(data);
+  aRenderer->SetDirty();
+  return true;
+}
+
 mozilla::UniquePtr<uint8_t[]> CanvasContext::GetImageBuffer(int32_t* aFormat) {
   gfxAlphaType any;
   RefPtr<gfx::SourceSurface> snapshot = GetSurfaceSnapshot(&any);

dom/webgpu/CanvasContext.h

@@ -54,6 +54,8 @@ class CanvasContext final : public nsICanvasRenderingContextInternal,
     return NS_OK;
   }
 
+  bool InitializeCanvasRenderer(nsDisplayListBuilder* aBuilder,
+                                layers::CanvasRenderer* aRenderer) override;
   mozilla::UniquePtr<uint8_t[]> GetImageBuffer(int32_t* aFormat) override;
   NS_IMETHOD GetInputStream(const char* aMimeType,
                             const nsAString& aEncoderOptions,