Bug 1573343 - Always create the back buffer sized to the widget's cli…

…ent size. r=mattwoodrow

This should be equivalent to what we had before but clearer.
Before this patch, we would reallocate the back buffer if
 - the back buffer was larger than the client size (meaning the window had shrunk) or
 - the invalid region didn't fit in the back buffer.
Whenever the window is resized, we have a frame where the invalid region covers
the entire window, and thus size would be equal clientSize in that frame.
So effectively, the back buffer size was always kept in sync with the client size.

Differential Revision: https://phabricator.services.mozilla.com/D41671

--HG--
extra : moz-landing-system : lando

widget/CompositorWidget.cpp

@@ -30,15 +30,14 @@ already_AddRefed<gfx::DrawTarget> CompositorWidget::GetBackBufferDrawTarget(
           ? gfx::SurfaceFormat::B8G8R8X8
           : gfx::SurfaceFormat::B8G8R8A8;
   gfx::IntSize size = aRect.ToUnknownRect().Size();
-  gfx::IntSize clientSize(GetClientSize().ToUnknownSize());
+  gfx::IntSize clientSize = Max(size, GetClientSize().ToUnknownSize());
 
   RefPtr<gfx::DrawTarget> target;
   // Re-use back buffer if possible
   if (mLastBackBuffer &&
       mLastBackBuffer->GetBackendType() == aScreenTarget->GetBackendType() &&
       mLastBackBuffer->GetFormat() == format &&
-      size <= mLastBackBuffer->GetSize() &&
-      mLastBackBuffer->GetSize() <= clientSize) {
+      mLastBackBuffer->GetSize() == clientSize) {
     target = mLastBackBuffer;
     target->SetTransform(gfx::Matrix());
     if (!aClearRect.IsEmpty()) {
@@ -48,7 +47,7 @@ already_AddRefed<gfx::DrawTarget> CompositorWidget::GetBackBufferDrawTarget(
                                   clearRect.Width(), clearRect.Height()));
     }
   } else {
-    target = aScreenTarget->CreateSimilarDrawTarget(size, format);
+    target = aScreenTarget->CreateSimilarDrawTarget(clientSize, format);
     mLastBackBuffer = target;
   }
   return target.forget();