As I felt I had made clear in my original post, I already uninstalled Start11 and confirmed the issue went away. When I reinstalled it, the issue was immediately happening again.
I had previously installed Start 11 over a year ago and haven't had any issues until this past week, at most, and only noticed it for the first time, yesterday. After completely uninstalling, restarting, and installing Start11, the exact same issue is happening again.
Here are the answers to your questions, in order:
1. Windows 11 Professional, version 25H2, Feature Pack 1000.26100.291.0
2. Start11 2.6.0.2
3. Enhanced taskbar is ON (and always has been).
4. Buttons are set to always combine (and have always been set that way)
5. I have no other programs installed that would interfere. I use DisplayFusion on another machine, entirely, but have never used it on this one.
I just tested turning off the Enhanced Taskbar and that fixed the issue. However, I have to have this turned on because I need my taskbar to auto-hide and it will only do so with Enhanced turned on. Also, as I said, I've always had that setting turned on and never had the issue before recently.
I also just tested the Enhanced Taskbar on another machine that was not having the issue, since that appears to be the difference between this machine and the others I have Start11 on. I have confirmed that the same issue happens on my other machine when I turn the enhancements on. Here's the info on that computer.
1. Windows 11 Professional, version 25H2, Feature Pack 1000.26100.291.0 (same as above)
2. Start11 2.6.0.2 (same as above)
3. Enhanced taskbar was not on, and issue wasn't happening. Happens now, when turned on.
4. Buttons are set to always combine when enhancements are on.
5. This computer does have DisplayFusion on it, but that's obviously not the source of the issue. I've seen zero issues using DisplayFusion with Start11 on this machine, in the past.
I hope that all helps. It looks like the issue is with the Enhanced Taskbar, possibly due to the recent Start11 update alone or possibly in conjunction with the recent Windows update (I heard there were Explorer issues).