Ben, if we also do the same thing for dragging tabs, when the tab is dragged into the split preview area, the overview windows in the overview moved to the opposite side of the screen, in this case: do we still show the new selector item (i.e., the item with the plus sign)?

See a recorded video that reposition the overview windows with the new selector item: https://drive.google.com/open?id=1koXqSVuf58CpBQZvKrRVhL5zZrJCN-xm

Ben, does it look good to you?

Really good question. I know it makes it more complicated, but I think we should take the opportunity to hide the indicator when the split side preview area is entered. It should re-appear if the user moves back to the center to cancel split. This way, there's no further animation when the user lets go.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fcab234783a396391ce97a62ec6c8849ebb8d01f

commit fcab234783a396391ce97a62ec6c8849ebb8d01f
Author: Xiaoqian Dai <xdai@chromium.org>
Date: Thu Aug 16 17:02:03 2018

When dragging a window around overview, reposition overview windows.

When dragging a window from caption area in tablet mode, overview will
open behind the dragged windows. When the dragged window is dragged into
the preview split area, we should update the overview grid's bounds and
also update the new selector item's visibility.

See recorded video:
https://drive.google.com/open?id=10gPVG1Wlgt4YDnZI_tPS2p6JJGYJ-ztl

Bug:  873292 
Change-Id: Ibc7028820bed11d61bdbb376fdc6723c26a0c262
Reviewed-on: https://chromium-review.googlesource.com/1176220
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583696}
[modify] https://crrev.com/fcab234783a396391ce97a62ec6c8849ebb8d01f/ash/wm/overview/overview_utils.h
[modify] https://crrev.com/fcab234783a396391ce97a62ec6c8849ebb8d01f/ash/wm/overview/window_grid.cc
[modify] https://crrev.com/fcab234783a396391ce97a62ec6c8849ebb8d01f/ash/wm/overview/window_grid.h
[modify] https://crrev.com/fcab234783a396391ce97a62ec6c8849ebb8d01f/ash/wm/splitview/split_view_controller_unittest.cc
[modify] https://crrev.com/fcab234783a396391ce97a62ec6c8849ebb8d01f/ash/wm/tablet_mode/tablet_mode_window_drag_delegate.cc