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 364 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));
+      // Always use the start point so the MacViews'

[tapted, 2016/04/06 09:38:51]

This affects all platforms, so we need to say why it's right to use
start_point_in_screen_ in general. Also it's typical in a comment not just to
say something will be wrong, but to say how it's wrong.

I think the gist of this is something like

// 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.

[themblsha, 2016/04/06 17:54:11]

Thanks, your suggestion is correct and succinct. 

+      // BridgedNativeWidget::RunMoveLoop() could position the window so the
+      // mouse would be directly on top of start_point_in_screen_. Otherwise the
+      // point by which the window is dragged will be wrong.
+      //
+      // The DetachToBrowserTabDragControllerTest.DragAll/0 test on MacViews
+      // will detect the inconsistencies.
+      RunMoveLoop(GetWindowOffset(start_point_in_screen_));
       return;
     }
   }
 
   if (ContinueDragging(point_in_screen) == Liveness::DELETED)
     return;
 }
 
 void TabDragController::EndDrag(EndDragReason reason) {
   TRACE_EVENT0("views", "TabDragController::EndDrag");
@@ skipping 1427 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;
 }