Dear Maintainer,

*** Reporter, please consider answering these questions, where appropriate ***

   * What led up to the situation?
     Start Thunderbird and minimize the window, then start an app using full
screen (in my case play video with mpv in full screen).
     When finished, try to recover the window by clicking on the icon in the
taskbar. There is a short flickering but no main window of Thunderbird shows
   * What exactly did you do (or not do) that was effective (or ineffective)?
     Thunderbird is still active and receive mails, but there is no window.
Close Thunderbird and start again is the only thing that helps.
   * What was the outcome of this action?
     After close and start again it was working normal.
   * What outcome did you expect instead?
     Restore window even after an app in full screen mode was running.