Fix high-DPI on Windows to make use of DIP scaling in WebKit.

ui/surface/accelerated_surface_win.cc

Index: ui/surface/accelerated_surface_win.cc
diff --git a/ui/surface/accelerated_surface_win.cc b/ui/surface/accelerated_surface_win.cc
index bcdf5f95d680ff52c1c1891cf00574bcc2514d1a..72d7929630c39e2a0c1b15a65874f0cab40507bd 100644
--- a/ui/surface/accelerated_surface_win.cc
+++ b/ui/surface/accelerated_surface_win.cc
@@ -23,6 +23,7 @@
 #include "base/threading/thread_restrictions.h"
 #include "base/time.h"
 #include "base/win/wrapped_window_proc.h"
+#include "ui/base/win/dpi.h"
 #include "ui/base/win/hwnd_util.h"
 #include "ui/gfx/rect.h"
 #include "ui/gl/gl_switches.h"
@@ -476,7 +477,19 @@ void AcceleratedPresenter::DoPresentAndAcknowledge(
   // If the window is a different size than the swap chain that is being
   // presented then drop the frame.
   gfx::Size window_size = GetWindowSize();
-  if (hidden_ && size != window_size) {
+#if defined(ENABLE_HIDPI)
+  // Check if the size mismatch is within allowable round off or truncation
+  // error.
+  gfx::Size dip_size = ui::win::ScreenToDIPSize(window_size);

[sky, 2013/01/28 20:56:35]

Does this ifdef mean GetWindowSize has different meanings on aura and windows?

[kevers, 2013/01/28 22:53:40]

The size mismatch on Windows with high-DPI enabled results from truncation
errors when converting between pixel and DIP.  Requiring an exact size match
resulted in dropping all frames when the size in pixels is not an integer
multiple of the size in DIP.  A mismatch is far more common with the fractional
scale factors for Windows (1.4 and 1.8 for Win8 and 1.25 and 1.5 for Win7).

Added the high-DPI specific test in order to get some benefit of frame dropping
to make a rapid resize feel smoother.

The method GetWindowSize returns the size in the client's coordinate space. 
Under views, with high-DPI enabled, it maps to screen pixels.

Not clear why we needed to drop the test on Windows-Aura.

+  gfx::Size pixel_size = ui::win::DIPToScreenSize(dip_size);
+  bool size_mismatch = abs(window_size.width() - size.width()) >
+      abs(window_size.width() - pixel_size.width()) ||
+      abs(window_size.height() - size.height()) >
+      abs(window_size.height() - pixel_size.height());
+#else
+  bool size_mismatch = size != window_size;
+#endif
+  if (hidden_ && size_mismatch) {
     TRACE_EVENT2("gpu", "EarlyOut_WrongWindowSize",
                  "backwidth", size.width(), "backheight", size.height());
     TRACE_EVENT2("gpu", "EarlyOut_WrongWindowSize2",