Removing a DCHECK in OverscrollWindowAnimation::OnOverscrollModeChange

content/browser/web_contents/aura/overscroll_window_animation.cc

 // Copyright 2015 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 "content/browser/web_contents/aura/overscroll_window_animation.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/i18n/rtl.h"
@@ skipping 103 matching lines @@
                                    ? -slide_window_bounds.width()
                                    : slide_window_bounds.width(),
                                0);
   } else {
     slide_window_bounds.Offset(base::i18n::IsRTL()
                                    ? slide_window_bounds.width() / 2
                                    : -slide_window_bounds.width() / 2,
                                0);
   }
 
-  DCHECK_EQ(overscroll_source_, OverscrollSource::NONE);

[mohsen, 2017/07/20 17:22:37]

Do you mean that this is not always true? If yes, when/why?

[mfomitchev, 2017/07/20 17:28:09]

See the CL that I linked in the description. Basically it's possible for the
overlay to send OnOvercrollModeChange(NONE, WEST) via OverscrollWindowDelegate
while the "cancel" animation is in progress, then for the overlay to be
destroyed, and then for the OverscrollController to send the same
OnOvercrollModeChange. Then this DCHECK will trigger.

Obscure scenarios like this is one of the reasons it would be great to replace
this with a simplified version :)

[mohsen, 2017/07/20 18:14:26]

Opps! Sorry, I missed the CL in the description! 

   overscroll_source_ = source;
   slide_window_ = new_direction == SLIDE_FRONT
                       ? delegate_->CreateFrontWindow(slide_window_bounds)
                       : delegate_->CreateBackWindow(slide_window_bounds);
   if (!slide_window_) {
     // Cannot navigate, do not start an overscroll gesture.
     direction_ = SLIDE_NONE;
     overscroll_source_ = OverscrollSource::NONE;
     return;
   }
@@ skipping 51 matching lines @@
 ui::Layer* OverscrollWindowAnimation::GetBackLayer() const {
   DCHECK(direction_ != SLIDE_NONE);
   if (direction_ == SLIDE_BACK) {
     DCHECK(slide_window_);
     return slide_window_->layer();
   }
   return delegate_->GetMainWindow()->layer();
 }
 
 }  // namespace content