While screencasting a tab, do not disable rendering updates while hidden.

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

Index: chrome/browser/ui/views/tabs/tab_drag_controller.cc
diff --git a/chrome/browser/ui/views/tabs/tab_drag_controller.cc b/chrome/browser/ui/views/tabs/tab_drag_controller.cc
index c3980924258555be4adeb146fc11318f6f8da8a6..79ce2d21fd85313d683d31ecac637e332c877638 100644
--- a/chrome/browser/ui/views/tabs/tab_drag_controller.cc
+++ b/chrome/browser/ui/views/tabs/tab_drag_controller.cc
@@ -489,6 +489,9 @@ void TabDragController::Drag(const gfx::Point& point_in_screen) {
       return;  // User hasn't dragged far enough yet.
 
     started_drag_ = true;
+    // Prevent the WebContents HWND from being hidden by any of the model
+    // operations performed during the drag.
+    source_dragged_contents()->SetCapturingContents(true);

[sky, 2013/02/04 15:02:25]

At this point the tab is very likely still in the tabstrip. You should only need
to set this once the tab has been detached (where this call was before). What
made you need to move it?

[miu, 2013/02/04 21:36:52]

Short story: It's the simplest solution that fixes the bug (mentioned in the
descr.) and avoids adding a lot of extra complicated logic to TabDragController.

In more detail, the bug is this: After a tab is detached during a drag, there
are multiple scenarios where Attach() will not be called after the drag has
ended.  This is bad because SetCapturingContents(false) won't be called. 
Example scenarios:

1. Whenever you detach a tab from the tab strip, and then release it such that
it creates a new browser window for itself.

2. Whenever you detach a tab from the tab strip, and then hit the Escape key.

So, I spent a lot of time looking for a way to correct these problems, but I was
adding a *lot* of complicated logic to TabDragController to handle all the edge
cases.  Furthermore, I felt it would be way too easy for a bug regression to
occur when future logic changes are made by others.

Therefore, I turned to looking for the single code path that is always executed
on drag start (and on stop).  While you're correct in that we only *need* to
call SetCapturingContents(true) once the tab has been detached, it's perfectly
harmless to call it right at the start of the drag.  IMHO, the code is so much
cleaner this way.

[sky, 2013/02/04 22:48:50]

None-the-less it's the *wrong* state. The tab is not yet detached. Additionally
if you have live tab dragging on (as aura does), the method shouldn't be
invoked.
TabDragController should have code paths the deal with all the case you care
about. Those cases should be fixed to do the right thing.

[miu, 2013/02/05 01:06:25]

Fixed.  This was all a bit confusing to me.  Would have been nice if the Aura
behavior was a separate TabDragController implementation entirely, since a lot
of concepts/assumptions are different.
Done.  My changes to TabDragController are only to fix exactly the cases that
were broken.  And, it goes without saying that I've extensively re-tested all of
this to make sure.  ;-)

     SaveFocus();
     Attach(source_tabstrip_, gfx::Point());
     if (detach_into_browser_ && static_cast<int>(drag_data_.size()) ==
@@ -1135,9 +1138,6 @@ void TabDragController::Attach(TabStrip* attached_tabstrip,
         drag_data_[i].contents->SetDelegate(NULL);
         drag_data_[i].original_delegate = NULL;
       }
-
-      // Return the WebContents to normalcy.
-      source_dragged_contents()->SetCapturingContents(false);
     }
 
     // Inserting counts as a move. We don't want the tabs to jitter when the
@@ -1223,11 +1223,6 @@ void TabDragController::Detach(ReleaseCapture release_capture) {
 
   mouse_move_direction_ = kMovedMouseLeft | kMovedMouseRight;
 
-  // Prevent the WebContents HWND from being hidden by any of the model
-  // operations performed during the drag.
-  if (!detach_into_browser_)
-    source_dragged_contents()->SetCapturingContents(true);
-
   std::vector<gfx::Rect> drag_bounds = CalculateBoundsForDraggedTabs(0);
   TabStripModel* attached_model = GetModel(attached_tabstrip_);
   std::vector<TabRendererData> tab_data;
@@ -1605,6 +1600,7 @@ void TabDragController::EndDragImpl(EndDragType type) {
     // enough to trigger a drag.
     if (started_drag_) {
       RestoreFocus();
+      source_dragged_contents()->SetCapturingContents(false);
       if (type == CANCELED)
         RevertDrag();
       else