I have a 49" ultrawide display and use DF to segment that space into 3 screens. Primary is in the center with a left and right screen. I use DF taskbars on the left and right displays, and configure those to show just the app icons, no start menu, tray, etc.

In beta 20, when I open the start menu on the primary (center) display, it opens to the far left of the left display. When I open the taskbar search, that is correctly positioned over the center (windows) taskbar.

Yeah, same here. This is been ongoing forever. I don't use this feature at all because of it.

Beta 21 is better, but still not as good as Beta 19 and all versions before that for my case. In 21, the menu opens up in the far left position (as if DF wasn't running and the Start button was over there) but then it jumps to the center (virtual) display's start button position (the Windows Start button, not a DF Start button)
In Beta 19 it would do this once, then remember the new position.
In Beta 21, it jumps everytime.

I can confirm the behavior described by Steve. (10.0 Beta 21, Windows 10)

Yes, exactly. Beta 19 did this the first time after login then not again. Beta 20 always did had it on the left. Beta 21 behaves as you describe every time.

Additionally, If the Start-Menu-Key on the keyboard is used, the menu stays on the left side.

Edit:
The Start Key behavior is not happening always. It depends somehow on currently maximized / minimized windows.
After manually minimizing all opend windows to the taskbar, startmenu will open left and stay there (windows key + D does not trigger this "error") .
If one window is active / maximized, the start menu opens left and jumps to the center afterwards.

Thanks for taking care of it.
Unfortunately, the bug is still present in Beta 22.

Correct. The start menu opens at very left side and jumps to the correct position after roughly 500ms.

Debug Info attached.

Issue solved in v10.0, Beta 23

Thank you for the excellent support!