Fix problems correctly invalidating/repainting when our updated paint rect fo...

chrome/browser/renderer_host/render_widget_host_view.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_H_
 #define CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_H_
 
 #include "base/gfx/native_widget_types.h"
 #include "base/shared_memory.h"
 #include "webkit/glue/webplugin.h"
@@ skipping 73 matching lines @@
   // Sets the cursor to the one associated with the specified cursor_type
   virtual void UpdateCursor(const WebCursor& cursor) = 0;
 
   // Indicates whether the page has finished loading.
   virtual void SetIsLoading(bool is_loading) = 0;
 
   // Enable or disable IME for the view.
   virtual void IMEUpdateStatus(int control, const gfx::Rect& caret_rect) = 0;
 
   // Informs the view that a portion of the widget's backing store was painted.
-  // The view should copy the given rect from the backing store of the render
-  // widget onto the screen.
+  // The view should ensure this gets copied to the screen.
+  //
+  // There are subtle performance implications here.  The RenderWidget gets sent
+  // a paint ack after this returns, so if the view only ever invalidates in
+  // response to this, then on Windows, where WM_PAINT has lower priority than
+  // events which can cause renderer resizes/paint rect updates, e.g.
+  // drag-resizing can starve painting; this function thus provides the view its
+  // main chance to ensure it stays painted and not just invalidated.  On the
+  // other hand, if this always blindly paints, then if we're already in the
+  // midst of a paint on the callstack, we can double-paint unnecessarily.
+  // (Worse, we might recursively call RenderWidgetHost::GetBackingStore().)
+  // Thus implementers should generally paint as much of |rect| as possible
+  // synchronously with as little overpainting as possible.
   virtual void DidPaintRect(const gfx::Rect& rect) = 0;
 
   // Informs the view that a portion of the widget's backing store was scrolled
   // by dx pixels horizontally and dy pixels vertically. The view should copy
   // the exposed pixels from the backing store of the render widget (which has
   // already been scrolled) onto the screen.
   virtual void DidScrollRect(
       const gfx::Rect& rect, int dx, int dy) = 0;
 
   // Notifies the View that the renderer has ceased to exist.
@@ skipping 27 matching lines @@
 
   // Whether the window can be activated. Autocomplete popup windows for example
   // cannot be activated.  Default is true.
   bool activatable_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostView);
 };
 
 #endif  // CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_H_