WIP - Gutterless resize on Windows

ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
index 48368f962f2f3433b76c5ad20746ad28f135a481..ce30eb0941a57cd0f7fbf70705825359e4a682f6 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
@@ -15,6 +15,7 @@
 #include "ui/base/cursor/cursor_loader_win.h"
 #include "ui/base/ime/input_method.h"
 #include "ui/base/win/shell.h"
+#include "ui/base/window_resize_helper.h"
 #include "ui/compositor/compositor_constants.h"
 #include "ui/compositor/paint_context.h"
 #include "ui/gfx/geometry/insets.h"
@@ -908,6 +909,10 @@ void DesktopWindowTreeHostWin::HandleWindowSizeChanged() {
   if (compositor())
     compositor()->SetScaleAndSize(compositor()->device_scale_factor(),
                                   compositor()->size());
+
+  // If the window size has changed and this is a visible window, we may
+  // want to try to immediately produce a frame.
+  MaybeWaitForFrame();
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -949,6 +954,36 @@ bool DesktopWindowTreeHostWin::IsModalWindowActive() const {
   return false;
 }
 
+void DesktopWindowTreeHostWin::MaybeWaitForFrame() {
+  if (!compositor())
+    return;
+
+  const gfx::Size target_size = compositor()->size();
+
+  if (!compositor()->IsVisible() ||
+      compositor()->last_swapped_frame_size() == target_size) {
+    return;
+  }
+
+  const int kPaintMsgTimeoutMS = 55;

[piman, 2015/12/15 07:15:36]

Any specific reason for 55 vs 50?
My gut feeling is that we should use the same value on all platforms, unless
there is a good reason not to.

+  const base::TimeTicks start_time = base::TimeTicks::Now();
+  const base::TimeTicks timeout_time =
+      start_time + base::TimeDelta::FromMilliseconds(kPaintMsgTimeoutMS);
+
+  ui::WindowResizeHelper* resize_helper = ui::WindowResizeHelper::Get();
+  for (base::TimeTicks now = start_time; now < timeout_time;
+       now = base::TimeTicks::Now()) {
+    if (!resize_helper->WaitForSingleTaskToRun(timeout_time - now))
+      return;  // Timeout.
+
+    // Since the UI thread is blocked, the size shouldn't change.
+    DCHECK(target_size == compositor()->size());
+    if (compositor()->last_swapped_frame_size() == target_size) {
+      return;  // Frame arrived.
+    }
+  }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // DesktopWindowTreeHost, public: