Improve fullscreen handling

[tnixeu]

I confirm (by marking "x" in the [ ] below: [x]):

• [x] This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/.
• [x] This doesn't reproduce on web browsers (such as in Chrome). If it does, issue reports go to the Mattermost Server repository.
• [x] I have read contributing guidelines.

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Summary

I accidentally enabled fullscreen and did not think of trying to toggle it.

Environment

• Operating System: Windows 10
• Mattermost Desktop App version: 4.71 and 4.7.2-rc3
• Mattermost Server version: 5.37.1

Steps to reproduce

• Enable fullscreen
• Forget about having enabled fullscreen
• Try moving window by using the title bar

Expected behavior

Have some indication that Mattermost is in fullscreen mode.

Observed behavior

There is no indication that the fullscreen mode is enabled, besides filling the full screen but also having a title bar. Not even a restart of the App or PC helped. I did not know that Mattermost now stays if fullscreen even when the App is restarted. Seems quite new: #1546

Possible fixes

Add some indication that Mattermost is in fullscreen mode would be helpful:

• Hide window icons or allow the maximizing button to disable fullscreen mode
• Allow dragging the title bar to disable fullscreen
• Add check mark to "Toggle Full Screen F11" in menu
• Show toast notification on startup that fullscreen is enabled
• Add setting to enable restarting with fullscreen enabled. This would allow people to know about this feature.

[devinbinnie]

@esethna @matthewbirtch Thoughts?

[esethna]

Already a ticket for this, but it needs UX. Agree that full screen mode feels kind of broken right now https://mattermost.atlassian.net/browse/MM-22955

cc// @matthewbirtch

[devinbinnie]

Closing as I think the full screen case in Windows works well enough, we've done some changes to resizing that should make sure the experience works well.