Bug 1258086 - Clear unbuffered SHM images with a 32-bit visual before drawing with the basic layer manager. r=lsalzman

MozReview-Commit-ID: 8XgVwvBDZ3X

--HG--
extra : rebase_source : ba0ed32c5b8241fa6bf6300a8fa97ad71a236c1a

widget/gtk/nsWindow.cpp

@@ -2282,6 +2282,17 @@ nsWindow::OnExposeEvent(cairo_t *cr)
     bool painted = false;
     {
       if (GetLayerManager()->GetBackendType() == LayersBackend::LAYERS_BASIC) {
+        GdkScreen *screen = gdk_window_get_screen(mGdkWindow);
+        if (GetTransparencyMode() == eTransparencyTransparent &&
+            layerBuffering == BufferMode::BUFFER_NONE &&
+            gdk_screen_is_composited(screen) &&
+            gdk_window_get_visual(mGdkWindow) ==
+            gdk_screen_get_rgba_visual(screen)) {
+          // If our draw target is unbuffered and we use an alpha channel,
+          // clear the image beforehand to ensure we don't get artifacts from a
+          // reused SHM image. See bug 1258086.
+          dt->ClearRect(Rect(boundsRect));
+        }
         AutoLayerManagerSetup setupLayerManager(this, ctx, layerBuffering);
         painted = listener->PaintWindow(this, region);
       }