MacViews: Implement Tab Dragging

chrome/browser/ui/views/tabs/tab_drag_controller.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/ui/views/tabs/tab_drag_controller.h"
 
 #include <math.h>
 #include <set>
 
 #include "base/auto_reset.h"
@@ skipping 372 matching lines @@
                           point_in_screen.x() -
                           mouse_offset_.x(), 0);
         widget->SetVisibilityChangedAnimationsEnabled(false);
         widget->Restore();
         widget->SetBounds(new_bounds);
         AdjustBrowserAndTabBoundsForDrag(last_tabstrip_width,
                                          point_in_screen,
                                          &drag_bounds);
         widget->SetVisibilityChangedAnimationsEnabled(true);
       }
-      RunMoveLoop(GetWindowOffset(point_in_screen));
+      // Use the start point to initialize the move loop, otherwise the point
+      // under the cursor will be offset by the drag threshold from where the
+      // user originally clicked, which could be entirely off the window.
+      RunMoveLoop(GetWindowOffset(start_point_in_screen_));

[sky, 2016/06/07 15:57:55]

I think the right thing here is to move the window by
point_in_screen-start_point_in_screen and then start the move loop using
point_in_screen.

       return;
     }
   }
 
   if (ContinueDragging(point_in_screen) == Liveness::DELETED)
     return;
 }
 
 void TabDragController::EndDrag(EndDragReason reason) {
   TRACE_EVENT0("views", "TabDragController::EndDrag");
@@ skipping 1403 matching lines @@
   // TODO(pkotwicz): Fix this properly (crbug.com/358482)
   for (auto* browser : *BrowserList::GetInstance()) {
     if (browser->tab_strip_model()->empty())
       exclude.insert(browser->window()->GetNativeWindow());
   }
 #endif
   base::WeakPtr<TabDragController> ref(weak_factory_.GetWeakPtr());
   *window = window_finder_->GetLocalProcessWindowAtPoint(screen_point, exclude);
   return ref ? Liveness::ALIVE : Liveness::DELETED;
 }