Hello,
Sorry to hear you are having issues. Need more information from your system.

1. Windows 11 full version and OS Build number. Please use Winver.exe.
2. Start11 full version number.
3. Post a video showing us these issue would help us understand it better.

Thank you.
Basj,
Stardock Community Assistant.

Please try the latest version V2.52 Beta. It should be available for you here: https://store.stardock.com/myaccount/products

Thank you,

Basj,
Stardock Community Assistant

It's been a couple of days since I installed v 2.51 as instructed. The issue remains. Today it's happening more frequently than usual. So, sorry, that wasn't a solution. 

I wish I could isolate the situation in which the problem rears its head. Of course, it's when (as I described) I move the mouse cursor within range of the area where the taskbar should fill when popped up, (IF I'd gone down that far, which I haven't,) so I'm right in the middle of being focused on what I do and it's all happened before I realize it. It's been hard to instantly recall how I got there, but I'll let you know if I do discover a pattern.

Bottom line, dysfunctional behavior continues...

Yes, you wrote: "Please try the latest version V2.52 Beta. It should be available for you here: https://store.stardock.com/myaccount/products"

I went to that link and installed the version I found there. I assumed that, since you asked me to try the beta, you gave me a link to the beta.

Where can I find Beta v. 2.52?

LOL, I see I was in one of my "old, blind man" moments. Thanks for indulging me. Downloading now.

[later]

Installed, I'll let you know how it goes. 

Thanks for your help!

Please try turning on this option.

Thank you,

Basj,
Stardock Community Assistant

Please check if you have disable Windows Autohide and only use Start11 autohide. Also, please check if there is no other application that also change how your Taskbar work. Such as Displayfusion, StartAllBack, etc

And finally, you should also try Cleanboot to rule out possibilities of conflict with other running application. Use this guide: https://forums.stardock.com/505872/start11-support-faq#cleanboot. As a reminder, please follow the guide with care.

Thank you,

Basj,
Stardock Community Assistant

I am trying to figure out what is different in your system that causing the issue. And from what I know, sometimes the conflict is one of the reason. Since you're too busy trying to figure out the issue specific to your system, I have forwarded your problem/question to Stardock Support Team for their assistance. Please keep an eye on this thread for any updates. We appreciate your feedback and patience.

Thank you.

Basj,
Stardock Community Assistant

I am experiencing this same behavior in Version 2.6. Windows Taskbar hide option is turned off, Reserve 1 Pixel option is turned on (though I've tried it both ways). If I force it to show by hovering the cursor at the bottom, and then let it auto-hide again, it no longer interferes. Switching desktops (Win+Ctrl+Right/Left) while it's hidden seems to cause it to "appear" invisibly. There may be other triggers, but that one is consistent.

I'm on Windows 11 Version 25H2, OS Build 26200.8117

The issue remains after installing 2.71. It seems to rear its ugly head the moment I navigate to or from a desktop that does not have a maximized application on the primary display.

By setting up a fresh instance of Start11, I have isolated the precise condition that gives rise to the issue, at least on my system.

When I turn rounded corners On for the taskbar, the behavior happens exactly as I outlined above. With rounded corners turned Off, the taskbar hides and shows as expected, no phantom appearances.

Thank you, Basj, for testing it out. I'm posting a video per your request, but first I do want to point that in your video the issue does appear, briefly, between the 7- and 8-second marks. When you switch over to the desktop with Settings and you move the pointer around at the bottom, for a moment the pointer changes to a vertical resize tool (double-headed arrow up-down) for no reason. You'll see it in my video below as well, but I thought it prudent to point it out.

In my video below, I have four desktops (conveniently labeled "Desktop 1", "Desktop 2", "Desktop 3", and "Desktop 4"). For the demo I'm switching between 2 and 3. Desktop 2 has the Start11 Settings app showing the state of the Rounded Corners option ("The Option").

0:00 - The Option is turned off for both Maximized and No Maximized options. Taskbar shows and hides as expected.
0:37 - Switch to Desktop 3, where the Paint app is maximized. Note the buttons on the right side of the status bar (Fit to Window and Zoom Level), which are hidden under the taskbar when the taskbar is visible. Here the taskbar is correctly hidden, and therefore I can interact with the buttons, and the buttons change state and show a tooltip when the pointer hovers over them.
0:59 - Switch back to Desktop 2, where I show no anomalies where the taskbar would be.
1:12 - I switch on The Option for No Maximized Windows, leaving The Option for Maximized Windows turned off.
1:28 - Switch back to Desktop 3. Note that the taskbar is (ostensibly) hidden, and the full maximized Paint window is visible, including aforementioned Fit-to-Window and Zoom-Level options.
1:32 - I hover over the Fit-to-Window and Zoom-Level buttons. They don't change state and no tooltip appears.
1:38 - Move the pointer up to where the top of the taskbar would be if it were visible. The pointer changes to a vertical resize tool, indicating readiness to resize the taskbar.
1:41 - Move the pointer to open the Zoom Level popup. Note the tooltip that appears is actually the tooltip for the System Tray Hidden Icons expander.
1:43 - Click now forces the taskbar to appear and the System Tray Hidden Icons pop up.
1:50 - Once the taskbar is hidden again, the Fit-to-Window and Zoom-Level buttons are accessible again.
1:58 - Switch back to Desktop 2, where I show that the vertical resizer now shows up in the empty part of the desktop where the top of taskbar would be. I also show tooltips that should only appear when the taskbar is visible, such as app previews when hovering over the taskbar app icons.
2:15 - I switch on The Option for Maximized Windows, to synchronize the two settings, and for the rest of the video I show that the issue has gone away.

In conclusion, the bug appears to be linked to switching the rounded-corner style between Maximized and No Maximized states when the two states don't have the same style. In further testing I have determined that simply maximizing or un-maximizing a window while the taskbar is hidden causes the issue to show itself.

I hope I have been sufficiently thorough in my demonstration.