Linux Aura: Use managed tab dragging when possible (Reland)

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 361 matching lines @@
 
     // On windows SaveFocus() may trigger a capture lost, which destroys us.
     {
       base::WeakPtr<TabDragController> ref(weak_factory_.GetWeakPtr());
       SaveFocus();
       if (!ref)
         return;
     }
     started_drag_ = true;
     Attach(source_tabstrip_, gfx::Point());
+    gfx::Point drag_point_in_screen = point_in_screen;
     if (static_cast<int>(drag_data_.size()) ==
         GetModel(source_tabstrip_)->count()) {
       if (was_source_maximized_ || was_source_fullscreen_) {
         did_restore_window_ = true;
         // When all tabs in a maximized browser are dragged the browser gets
         // restored during the drag and maximized back when the drag ends.
         views::Widget* widget = GetAttachedBrowserWidget();
         const int last_tabstrip_width = attached_tabstrip_->GetTabAreaWidth();
         std::vector<gfx::Rect> drag_bounds = CalculateBoundsForDraggedTabs();
         OffsetX(GetAttachedDragPoint(point_in_screen).x(), &drag_bounds);
         gfx::Rect new_bounds(CalculateDraggedBrowserBounds(source_tabstrip_,
                                                            point_in_screen,
                                                            &drag_bounds));
         new_bounds.Offset(-widget->GetRestoredBounds().x() +
                           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);
       } else {
+#if !defined(OS_LINUX) || defined(OS_CHROMEOS)
         // The user has to move the mouse some amount of pixels before the drag
         // starts. Offset the window by this amount so that the relative offset
         // of the initial location is consistent. See crbug.com/518740
         views::Widget* widget = GetAttachedBrowserWidget();
         gfx::Rect bounds = widget->GetWindowBoundsInScreen();
         bounds.Offset(point_in_screen.x() - start_point_in_screen_.x(),
                       point_in_screen.y() - start_point_in_screen_.y());
         widget->SetBounds(bounds);
+#else
+        // Linux does not need the window offset hack above because all drag
+        // implementations move windows relative to a passed-in cursor position
+        // instead of the implicit current cursor position.
+        drag_point_in_screen = start_point_in_screen_;

[sky, 2016/10/12 15:58:33]

I'm confused by this. If you use the original starting location, then don't you
miss the kMinimumDragDistance?

Also, in this context 'hack' is rather negative and the above code isn't a hack.
It's there to ensure specific behavior.

[Tom (Use chromium acct), 2016/10/12 18:08:27]

No, because I removed the widget->SetBounds above.

The reason I needed to do this is the SetBounds eventually results in a request
to move the window sent to the window manager.  The WM would then snap the
window to within the screen bounds, and we would then request the WM move the
window in RunMoveLoop with a stale starting point.
Sorry, removed.  I said that because that code was added to overcome a
limitation only on Windows platforms.

+#endif
       }
-      RunMoveLoop(GetWindowOffset(point_in_screen));
+      RunMoveLoop(GetWindowOffset(drag_point_in_screen));
       return;
     }
   }
 
   if (ContinueDragging(point_in_screen) == Liveness::DELETED)
     return;
 }
 
 void TabDragController::EndDrag(EndDragReason reason) {
   TRACE_EVENT0("views", "TabDragController::EndDrag");
@@ skipping 1412 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;
 }