[Help Wanted] Focus not set back to the next top-level window after closing Mattermost app window

[jasonblais]

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• This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/.
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Summary
Focus not set back to the next top-level window after closing Mattermost app window

Steps to reproduce

Original report here: #233
Reporter: @nmatt

Windows 10, Mattermost server v3.6.2, Mattermost desktop v3.5.0

1. Launch Mattermost desktop 3.5.0
2. Launch some other application
3. Alt+Tab back to Mattermost desktop
4. Close Mattermost desktop using keyboard or Close button
   => Focus is not back on the other application (but is still on the now invisible Mattermost desktop application)

Expected behavior

Focus is set back to the next top-level window

Observed behavior

Focus is not back on the other application (but is still on the now invisible Mattermost desktop application)

[jasonblais]

Might be related to #431.

[jasonblais]

Resolved by #464