bug 627628 - Throttle the dispatching of starved paints so that time is allowed for the processing of input events between each starved paint. r=roc

This helps maintain responsiveness in cases where input events are being generated very rapidly or where painting is extremely expensive.

widget/windows/nsWindow.cpp

@@ -3757,9 +3757,11 @@ void nsWindow::DispatchPendingEvents()
     --recursionBlocker;
   }
 
-  // Quickly check to see if there are any
-  // paint events pending.
-  if (::GetQueueStatus(QS_PAINT)) {
+  // Quickly check to see if there are any paint events pending,
+  // but only dispatch them if it has been long enough since the
+  // last paint completed.
+  if (::GetQueueStatus(QS_PAINT) &&
+      ((TimeStamp::Now() - mLastPaintEndTime).ToMilliseconds() >= 50)) {
     // Find the top level window.
     HWND topWnd = WinUtils::GetTopLevelHWND(mWnd);
 

widget/windows/nsWindow.h

@@ -61,6 +61,8 @@
 #include "nsWindowDbg.h"
 #include "cairo.h"
 #include "nsITimer.h"
+#include "mozilla/TimeStamp.h"
+
 #ifdef CAIRO_HAS_D2D_SURFACE
 #include "gfxD2DSurface.h"
 #endif
@@ -94,6 +96,8 @@ class imgIContainer;
 
 class nsWindow : public nsBaseWidget
 {
+  typedef mozilla::TimeStamp TimeStamp;
+  typedef mozilla::TimeDuration TimeDuration;
   typedef mozilla::widget::WindowHook WindowHook;
 #if MOZ_WINSDK_TARGETVER >= MOZ_NTDDI_WIN7
   typedef mozilla::widget::TaskbarWindowPreview TaskbarWindowPreview;
@@ -611,6 +615,10 @@ protected:
   bool                  mHasTaskbarIconBeenCreated;
 #endif
 
+  // The point in time at which the last paint completed. We use this to avoid
+  //  painting too rapidly in response to frequent input events.
+  TimeStamp mLastPaintEndTime;
+
 #ifdef ACCESSIBILITY
   static BOOL           sIsAccessibilityOn;
   static HINSTANCE      sAccLib;

widget/windows/nsWindowGfx.cpp

@@ -605,6 +605,7 @@ bool nsWindow::OnPaint(HDC aDC, PRUint32 aNestingLevel)
   }
 
   mPaintDC = nsnull;
+  mLastPaintEndTime = TimeStamp::Now();
 
 #if defined(WIDGET_DEBUG_OUTPUT)
   if (debug_WantPaintFlashing())