Reset scroll and scale while toggling "Request desktop site"

third_party/WebKit/Source/core/loader/FrameLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights
  * reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2008 Alp Toker <alp@atoker.com>
  * Copyright (C) Research In Motion Limited 2009. All rights reserved.
  * Copyright (C) 2011 Kris Jordan <krisjordan@gmail.com>
  * Copyright (C) 2011 Google Inc. All rights reserved.
@@ skipping 1087 matching lines @@
   }
   if (!NeedsHistoryItemRestore(load_type))
     return;
   HistoryItem* history_item = document_loader_->GetHistoryItem();
   if (!history_item || !history_item->DidSaveScrollOrScaleState())
     return;
 
   bool should_restore_scroll =
       history_item->ScrollRestorationType() != kScrollRestorationManual;
   bool should_restore_scale = history_item->PageScaleFactor();
+  if (!frame_->GetSettings()->GetViewportMetaEnabled()) {

[aelias_OOO_until_Jul13, 2017/05/25 02:56:08]

Unfortunately, there are indeed side effects:
1. On other Chrome platforms this setting is always false, because viewport meta
is mobile-specific feature.  So this is universally disabling scroll/scale
restore on non-Android.
2. Even if this were wrapped in "#if OS(ANDROID)" (which is discouraged pattern
anyway), we will also see the problem that further navigations starting from the
desktop site will no longer restore scroll/scale properly.

So this needs to be done conditionally that the *last* navigation was a special
RDS reload.

[jaebaek, 2017/05/25 04:10:34]

Does it mean that I must add a parameter which let the system know the reload is
actually from RDS and set it as TRUE at the moment when
NavigationControllerAndroid::SetUseDesktopUserAgent() is called (i.e., RDS
button is clicked)?

In the case, the parameter definitely will sent to settings (e.g., we can get it
by calling frame_->GetSettings()->GetRDSEnabled() here).

+    should_restore_scroll = false;
+    should_restore_scale = false;
+  }
 
   // This tries to balance:
   // 1. restoring as soon as possible
   // 2. not overriding user scroll (TODO(majidvp): also respect user scale)
   // 3. detecting clamping to avoid repeatedly popping the scroll position down
   //    as the page height increases
   // 4. ignore clamp detection if we are not restoring scroll or after load
   //    completes because that may be because the page will never reach its
   //    previous height
   bool can_restore_without_clamping =
@@ skipping 594 matching lines @@
   // TODO(japhet): This is needed because the browser process DCHECKs if the
   // first entry we commit in a new frame has replacement set. It's unclear
   // whether the DCHECK is right, investigate removing this special case.
   bool replace_current_item = load_type == kFrameLoadTypeReplaceCurrentItem &&
                               (!Opener() || !request.Url().IsEmpty());
   loader->SetReplacesCurrentHistoryItem(replace_current_item);
   return loader;
 }
 
 }  // namespace blink