I have had this recurring problem with Thunderbird, and only with Thunderbird. I have my Windows 10 taskbar set to auto-hide, so that it reappears when I hover the mouse over its location. This works for all programs but Thunderbird. When I have Thunderbird in expanded (full screen) window mode, hovering over the taskbar area does not make the taskbar reappear. The only way I can see it is to minimize the Thunderbird window. I had found a solution for this that worked for months, but I can't find it now. Today I found this archived topic: https://support.mozilla.org/en-US/questions/1271091 I set all the options as recommended in this article, although some of them seem irrelevant. I was so pleased that it solved the problem. Except that two hours later, the problem returned. I hadn't restarted the computer in the meantime, although the screen had turned off automatically in the meantime. I checked that all the settings were still as recommended in the article. I know this has been a recurring issue with Thunderbird, because it's been bothering me off and on for years, and I always see other people are bothered by it. (I've also seen complaints about this happening with Firefox, which I no longer use.) Usually I see the issue blamed on Windows, but it doesn't happen with Microsoft Office, any of several browsers, my photo editing software, Netflix, Youtube, or indeed with no other program but Thunderbird. It's hard to believe the fault lies with Windows. I could work around it by not using Thunderbird in fullscreen window mode, but that means the two window modes are practically the same size, and I really do sometimes want a smaller email window and sometimes a larger window. I would like to be able to shift between the two modes without losing my Windows taskbar. The archived article referenced above has been marked as "Solved", although I beg to differ.