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Bug 1592739 - Make nsChildView::WidgetPaintsBackground() return true. r=tnikkel

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On macOS, the OS window always comes with an opaque background for top level
windows. This is the case even if Gecko determines the root element of the
window to be transparent: Ever since bug 1162649, nsChildView/nsCocoaWindow
ignore calls to SetTransparencyMode for top level windows and always stay opaque.

Returning true from nsChildView::WidgetPaintsBackground() lets us indicate that
we do not need an opaque backstop color to be added at the bottom of the display
list. This backstop color would interfere with vibrant -moz-appearance rendering
under the new vibrancy model.
WidgetPaintsBackground() is only called in one place, in ComputeBackstopColor():

```
nscolor PresShell::ComputeBackstopColor(nsView* aDisplayRoot) {
  nsIWidget* widget = aDisplayRoot->GetWidget();
  if (widget && (widget->GetTransparencyMode() != eTransparencyOpaque ||
                 widget->WidgetPaintsBackground())) {
    // Within a transparent widget, so the backstop color must be
    // totally transparent.
    return NS_RGBA(0, 0, 0, 0);
  }
  // Within an opaque widget (or no widget at all), so the backstop
  // color must be totally opaque. The user's default background
  // as reported by the prescontext is guaranteed to be opaque.
  return GetDefaultBackgroundColorToDraw();
}
```

On Windows 7, the widget returns eTransparencyBorderlessGlass from
GetTransparencyMode(), which also avoids the backstop color.

Differential Revision: https://phabricator.services.mozilla.com/D51458

--HG--
extra : moz-landing-system : lando
This commit is contained in:
Markus Stange 2019-11-05 18:47:21 +00:00
parent 691e7d64a1
commit 4c8859d2b4
1 changed files with 2 additions and 0 deletions
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2
widget/cocoa/nsChildView.h
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@ -487,6 +487,8 @@ class nsChildView final : public nsBaseWidget {
virtual void PrepareWindowEffects() override; virtual void PrepareWindowEffects() override;
virtual void CleanupWindowEffects() override; virtual void CleanupWindowEffects() override;
virtual bool WidgetPaintsBackground() override { return true; }
virtual void AddWindowOverlayWebRenderCommands( virtual void AddWindowOverlayWebRenderCommands(
mozilla::layers::WebRenderBridgeChild* aWrBridge, mozilla::wr::DisplayListBuilder& aBuilder, mozilla::layers::WebRenderBridgeChild* aWrBridge, mozilla::wr::DisplayListBuilder& aBuilder,
mozilla::wr::IpcResourceUpdateQueue& aResourceUpdates) override; mozilla::wr::IpcResourceUpdateQueue& aResourceUpdates) override;