Fix a screen capture issue on retina macOS devices.

webrtc/modules/desktop_capture/screen_capturer_mac.mm

Index: webrtc/modules/desktop_capture/screen_capturer_mac.mm
diff --git a/webrtc/modules/desktop_capture/screen_capturer_mac.mm b/webrtc/modules/desktop_capture/screen_capturer_mac.mm
index f11b36d2d2001d6fd0079d8d9464a1dcfd7c89fb..82d2f9a7ebe18388c96b89f08b65c87b8b15b19b 100644
--- a/webrtc/modules/desktop_capture/screen_capturer_mac.mm
+++ b/webrtc/modules/desktop_capture/screen_capturer_mac.mm
@@ -304,6 +304,10 @@ class ScreenCapturerMac : public DesktopCapturer {
   bool RegisterRefreshAndMoveHandlers();
   void UnregisterRefreshAndMoveHandlers();
 
+  void ScreenUpdateMove(CGFloat delta_x,
+                        CGFloat delta_y,
+                        size_t count,
+                        const CGRect* rect_array);
   void ScreenRefresh(CGRectCount count, const CGRect *rect_array);
   void ReleaseBuffers();
 
@@ -955,10 +959,18 @@ bool ScreenCapturerMac::RegisterRefreshAndMoveHandlers() {
 
           size_t count = 0;
           const CGRect* rects = CGDisplayStreamUpdateGetRects(
-              updateRef, kCGDisplayStreamUpdateDirtyRects, &count);

[Sergey Ulanov, 2016/12/20 00:47:04]

FWIW kCGDisplayStreamUpdateDirtyRects is document as "The union of both
rectangles that were redrawn and rectangles that were moved.". It's ambiguous. I
assumed that it would return both old an new location of the moved rects, but
maybe it's not the way it's implemented. What rects do you see returned from
this call when a window is moved?

+              updateRef, kCGDisplayStreamUpdateMovedRects, &count);
+          if (count != 0) {
+            CGFloat dx = 0;
+            CGFloat dy = 0;
+            CGDisplayStreamUpdateGetMovedRectsDelta(updateRef, &dx, &dy);
+            ScreenUpdateMove(dx, dy, count, rects);
+          }
+
+          count = 0;
+          rects = CGDisplayStreamUpdateGetRects(
+              updateRef, kCGDisplayStreamUpdateRefreshedRects, &count);
           if (count != 0) {
-            // According to CGDisplayStream.h, it's safe to call
-            // CGDisplayStreamStop() from within the callback.
             ScreenRefresh(count, rects);
           }
         };
@@ -984,6 +996,23 @@ void ScreenCapturerMac::UnregisterRefreshAndMoveHandlers() {
   display_stream_manager_->UnregisterActiveStreams();
 }
 
+void ScreenCapturerMac::ScreenUpdateMove(CGFloat delta_x,
+                                         CGFloat delta_y,
+                                         size_t count,

[Sergey Ulanov, 2016/12/20 00:47:04]

CGRectCount?

[erikchen, 2016/12/20 03:11:24]

Done.

+                                         const CGRect* rect_array) {
+  // Translate |rect_array| to identify the move's destination. Then take the
+  // union of that and the original rect, since we need to update both the
+  // previous position and the new position.
+  CGRect refresh_rects[count];
+  for (CGRectCount i = 0; i < count; ++i) {
+    CGRect new_rect = CGRectOffset(rect_array[i], delta_x, delta_y);
+    refresh_rects[i] = CGRectUnion(new_rect, refresh_rects[i]);

[Sergey Ulanov, 2016/12/20 00:47:04]

I don't think we need rect union here. It would expand the region that needs to
be captured more than necessary. Instead I suggest calling ScreenRefresh()
twice: first with the original rect_array and then with with delta_x/y added

Also I would expect that when a window is moved the content that's not longer
covered by the window should be refreshed as part of the same notification, i.e.
it should be returned by kCGDisplayStreamUpdateRefreshedRects.

[erikchen, 2016/12/20 03:11:24]

Done.

+  }
+
+  // Currently we just treat move events the same as refreshes.
+  ScreenRefresh(count, refresh_rects);
+}
+
 void ScreenCapturerMac::ScreenRefresh(CGRectCount count,
                                       const CGRect* rect_array) {
   if (screen_pixel_bounds_.is_empty())