Terminating the presentation when disconnecting the display sounds good to me.

+btolsch, +amyroberts, +johnpallett, do you have thoughts?

I could also see it just being minimized/un-fullscreened to save the presentation state, but terminating the presentation might be cleaner in general.

I'd lean against terminating the presentation, unless we get some specific user complaints about keeping it around.  Some thoughts:

- In general we don't want to close browser windows in circumstances where the user might not expect it, as they may have unsaved state and/or some document the user was actually intending to use.

- If the wired display was the only presentation screen available to the user, there will be no way for the user to get the content back by initiating a new presentation.

- Or, if the user closed the tab that initiated the presentation, they will have to navigate back and start it again.

- Perhaps the user is switching screens/cables and wants to redirect the existing presentation to the new screen, instead of terminating it.

+1 to Brandon's suggestion to minimizing the window (so it doesn't interfere with what the user is doing).

FWIW desktop OSes automatically migrate existing desktop windows to the primary display when a secondary display is uplugged.
 

"desktop OSes automatically migrate existing desktop windows to the primary display when a secondary display is uplugged." 

This pattern makes moving the window to the remaining display feel the most expected IMO. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/38dbe75ee11110c5cf146472f7e5f871e521b648

commit 38dbe75ee11110c5cf146472f7e5f871e521b648
Author: Takumi Fujimoto <takumif@chromium.org>
Date: Thu Mar 01 19:34:16 2018

[Local Screen Casting] Exit fullscreen when presentation display is removed

While presenting to a local display, if we remove the display with a
fullscreen presentation, the presentation can move to the display with
the controlling window while still being fullscreen and cover the
controlling window. This CL makes the presentation window exit
fullscreen in such cases.

Bug:  805139 
Change-Id: I02c4234d95ee9477683550fdad454e1aee6b460b
Reviewed-on: https://chromium-review.googlesource.com/902889
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540256}
[modify] https://crrev.com/38dbe75ee11110c5cf146472f7e5f871e521b648/chrome/browser/media/router/providers/wired_display/wired_display_media_route_provider.cc
[modify] https://crrev.com/38dbe75ee11110c5cf146472f7e5f871e521b648/chrome/browser/media/router/providers/wired_display/wired_display_media_route_provider_unittest.cc
[modify] https://crrev.com/38dbe75ee11110c5cf146472f7e5f871e521b648/chrome/browser/media/router/providers/wired_display/wired_display_presentation_receiver.h
[modify] https://crrev.com/38dbe75ee11110c5cf146472f7e5f871e521b648/chrome/browser/ui/media_router/presentation_receiver_window.h
[modify] https://crrev.com/38dbe75ee11110c5cf146472f7e5f871e521b648/chrome/browser/ui/media_router/presentation_receiver_window_controller.cc
[modify] https://crrev.com/38dbe75ee11110c5cf146472f7e5f871e521b648/chrome/browser/ui/media_router/presentation_receiver_window_controller.h

Changed the behavior such that the presentation would be shown on the remaining display in windowed mode.