[Help Wanted] Mattermost desktop app icon appears in the "classic" Alt+Tab switcher list after app window is closed

[jasonblais]

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• This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/.
• This doesn't reproduce on web browsers (such as in Chrome). If it does, issue reports go to the Mattermost Server repository.
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Summary
Mattermost desktop app icon appears in the "classic" Alt+Tab switcher list after app window is closed

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 (to ensure there will be something else to show in the Alt+Tab switcher)
3. Alt+Tab back to Mattermost desktop
4. Close Mattermost desktop using keyboard or Close button
   Invoke the classic Alt+Tab switcher as follows:
5. Press and hold the left Alt key
6. Press and release the right Alt key (while still holding the left Alt key)
7. Press the Tab key (while still holding the left Alt key)

Expected behavior
Mattermost desktop app icon not in the classic Alt+Tab switcher list.

Observed behavior

The icon of the Mattermost desktop application is still in the classic Alt+Tab switcher list. It only disappears from it after having switched to a different application.

[jasonblais]

Resolved by #464