Good catch. Do we currently show different overview screens for each display in a multi-display setup (i.e. with the windows from each display showing up in overview on the display they're currently placed on - and nowhere else)?

If this is the case, we can just keep overview blank on any display with no windows (but still with blurred background and sheld). If there are no open windows on any display, we can show the "no recent items" label on a single display. I haven't used CrOS on multi-displays, so I don't know exactly how it works, but I would put the "no recent items" on the display that has the shelf, or is set as a primary, or if neither of those things are how it works on CrOS, then I'd put it on the laptop/tablet display, if one exists.

Yes, on multi-display, each display has a window grid. The windows associated with each display are shown on that grid, and only that grid. The text box is always on the primary display, even if the primary display has no windows for overview, but highlights windows across all grids.

If we enter overview mode currently without some windows, there is no background (no blurred background and shield). In tablet mode, multi display is not supported (extra displays become mirrors), so this is not a problem.

I'll add the "no recent items" on the primary display for now, and add the blurred background and shield for displays with no windows. We'll see if any other cases pop up.

Awesome, thanks Sammie! And thanks for the explanation.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2

commit 05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2
Author: Sammie Quon <sammiequon@google.com>
Date: Thu Mar 22 01:03:08 2018

overview: Modify how multi display overview works for new ui.

"No recent items" only shows up on the primary window grid now.
If all the windows on a grid are closed, the grid is still kept alive
until all windows on all grids are closed, so that the wallpaper blur
and opacity change are still seen.

Previously, the grid would destroy as soon as all its items were closed,
removing the blur and fade from that display, but the system was still
in overview mode because another grid was active.

If old UI is enabled, the behaviour is unchanged.

Test: WindowSelectorTest.OverviewNoWindowsIndicatorMultiDisplay
Bug:  804979 
Change-Id: I2ed9d4d13301a14ddf7a2ed39a299a5b8183eb2f
Reviewed-on: https://chromium-review.googlesource.com/967367
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544931}
[modify] https://crrev.com/05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2/ash/wm/overview/window_grid.cc
[modify] https://crrev.com/05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2/ash/wm/overview/window_grid.h
[modify] https://crrev.com/05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2/ash/wm/overview/window_selector.cc
[modify] https://crrev.com/05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2/ash/wm/overview/window_selector_item.cc
[modify] https://crrev.com/05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2/ash/wm/overview/window_selector_unittest.cc
[modify] https://crrev.com/05e32b12a408b22f5fd5ecd036fbc541b8ffb8b2/testing/buildbot/filters/mash.ash_unittests.filter