Bug 1674473 [Linux] SW-WR Clear background when painting popup windows, r=lsalzman

Clear alpha from buffer before painting transparent windows by SW Webrender to avoid rendering artifacts.

Differential Revision: https://phabricator.services.mozilla.com/D102189

gfx/webrender_bindings/RenderCompositorSWGL.cpp

@@ -61,6 +61,7 @@ bool RenderCompositorSWGL::AllocateMappedBuffer() {
   if (!mDT) {
     return false;
   }
+  mWidget->ClearBeforePaint(mDT, mRegion);
   // Attempt to lock the underlying buffer directly from the draw target.
   // Verify that the size at least matches what the widget claims and that
   // the format is BGRA8 as SWGL requires.

widget/CompositorWidget.h

@@ -163,6 +163,13 @@ class CompositorWidget {
    */
   virtual bool NeedsToDeferEndRemoteDrawing() { return false; }
 
+  /**
+   * Some widgets (namely Gtk) may need clean up underlying surface
+   * before painting to draw transparent objects correctly.
+   */
+  virtual void ClearBeforePaint(RefPtr<gfx::DrawTarget> aTarget,
+                                const LayoutDeviceIntRegion& aRegion) {}
+
   /**
    * Called when shutting down the LayerManager to clean-up any cached
    * resources.

widget/gtk/GtkCompositorWidget.cpp

@@ -14,6 +14,8 @@
 namespace mozilla {
 namespace widget {
 
+using namespace gfx;
+
 GtkCompositorWidget::GtkCompositorWidget(
     const GtkCompositorWidgetInitData& aInitData,
     const layers::CompositorOptions& aOptions, nsWindow* aWindow)
@@ -125,5 +127,20 @@ void GtkCompositorWidget::SetEGLNativeWindowSize(
 }
 #endif
 
+void GtkCompositorWidget::ClearBeforePaint(
+    RefPtr<gfx::DrawTarget> aTarget, const LayoutDeviceIntRegion& aRegion) {
+  // We need to clear target buffer alpha values of popup windows as
+  // SW-WR paints with alpha blending (see Bug 1674473).
+  if (mWidget->IsPopup()) {
+    for (auto iter = aRegion.RectIter(); !iter.Done(); iter.Next()) {
+      LayoutDeviceIntRect r = iter.Get();
+      aTarget->FillRect(gfx::Rect(r.x, r.y, r.width, r.height),
+                        ColorPattern(DeviceColor(0, 0, 0, 0)),
+                        DrawOptions(1.0f, CompositionOp::OP_SOURCE));
+    }
+    aTarget->Flush();
+  }
+}
+
 }  // namespace widget
 }  // namespace mozilla

widget/gtk/GtkCompositorWidget.h

@@ -61,6 +61,9 @@ class GtkCompositorWidget : public CompositorWidget,
   EGLNativeWindowType GetEGLNativeWindow();
   int32_t GetDepth();
 
+  void ClearBeforePaint(RefPtr<gfx::DrawTarget> aTarget,
+                        const LayoutDeviceIntRegion& aRegion) override;
+
 #if defined(MOZ_X11)
   Display* XDisplay() const { return mXDisplay; }
   Window XWindow() const { return mXWindow; }

widget/gtk/nsWindow.cpp

@@ -1248,10 +1248,12 @@ void nsWindow::Move(double aX, double aY) {
   }
 }
 
-bool nsWindow::IsWaylandPopup() {
-  return !mIsX11Display && mIsTopLevel && mWindowType == eWindowType_popup;
+bool nsWindow::IsPopup() {
+  return mIsTopLevel && mWindowType == eWindowType_popup;
 }
 
+bool nsWindow::IsWaylandPopup() { return !mIsX11Display && IsPopup(); }
+
 void nsWindow::HideWaylandTooltips() {
   while (gVisibleWaylandPopupWindows) {
     nsWindow* window =

widget/gtk/nsWindow.h

@@ -298,6 +298,7 @@ class nsWindow final : public nsBaseWidget {
   GtkWidget* GetGtkWidget() { return mShell; }
   nsIFrame* GetFrame();
   bool IsDestroyed() { return mIsDestroyed; }
+  bool IsPopup();
   bool IsWaylandPopup();
   bool IsPIPWindow() { return mIsPIPWindow; };