Bug 806029: Have remote content drop their buffers when they're hidden. r=mattwoodrow sr=roc

layout/base/nsPresShell.cpp

@@ -9070,6 +9070,38 @@ PresShell::SetIsActive(bool aIsActive)
     }
   }
 #endif
+
+  // We have this odd special case here because remote content behaves
+  // differently from same-process content when "hidden".  In
+  // desktop-type "browser UIs", hidden "tabs" have documents that are
+  // part of the chrome tree.  When the tabs are hidden, their content
+  // is no longer part of the visible document tree, and the layers
+  // for the content are naturally released.
+  //
+  // Remote content is its own top-level tree in its subprocess.  When
+  // it's "hidden", there's no transaction in which the document
+  // thinks it's not visible, so layers can be retained forever.  This
+  // is problematic when those layers uselessly hold on to precious
+  // resources like directly texturable memory.
+  //
+  // PresShell::SetIsActive() is the first C++ entry point at which we
+  // (i) know that our parent process wants our content to be hidden;
+  // and (ii) has easy access to the TabChild.  So we use this
+  // notification to signal the TabChild to drop its layer tree and
+  // stop trying to repaint.
+  if (TabChild* tab = GetTabChildFrom(this)) {
+    if (aIsActive) {
+      tab->MakeVisible();
+      if (nsIFrame* root = mFrameConstructor->GetRootFrame()) {
+        FrameLayerBuilder::InvalidateAllLayersForFrame(
+          nsLayoutUtils::GetDisplayRootFrame(root));
+        root->SchedulePaint();
+      }
+    } else {
+      tab->MakeHidden();
+    }
+  }
+
   return rv;
 }