When reporting an issue, please try to reproduce the issue with just Dominos and Dominos_Config running. Dominos reuses a good chunk of the stock UI, and other popular addons, like Move Anything, like to take control of those parts, too.

Addon Version
9.0.28-beta2-BCC

World of Warcraft Version
Burning Crusade Classic

To Reproduce - Configure your Menu Settings > Layout to "1" Column so that it is completely vertical. Pressing the Buttons or using Keybinds to open them, you will notice a small graphical issue when doing so. It goes away once the pressing sequence is completed, so I can't really record or post a screenshot unless it was a GIF or Video, so the best I can do is show how my configuration looks as well as my options I have configured. Any more questions just let me know - thanks again for the great addon!

In particular, it seems mostly prevalent with the Quest Log button, and happens when I press every button including the Quest Log button or any of the associated hotkeys.

For now, I think this is the best I can do.

With 9.0.29 it looks a bit better, but now it has introduced what I would consider as "flicker" when clicking the buttons or pressing the keybinds, and the issue is still present - but happens "faster"

I've reduced the refresh time on my end for that. Its a weird artifact of how the Blizzard UI refreshes the talent/quest button ordering whenever you press a button.