This should be a fix for this issue, the details are quoted here:

• When we open the palette popup, it tries to use some space of the "workarea" (width/2, height*2/3)
• That "workarea" is given by a fit_bounds() function
• On Linux/X11, fit_bounds() gets the workarea from the _NET_WORKAREA property of the root window, which it looks like includes the workarea of all monitors/screens (because the property doesn't depend on a specific screen). I think here is the issue in our case, we should get the workarea of one specific screen in this case (as it does on Windows and macOS).

This issue was previously an item of #2777

Basically we want to get the area that will be used if we maximize the current window in its own screen. From https://specifications.freedesktop.org/wm-spec/wm-spec-1.3.html#idm45682922243328 we can find:

With the introduction of the Xinerama extension in X11 R6.4, maximization has become more involved. Xinerama allows a screen to span multiple monitors in a freely configurable geometry. In such a setting, maximizing a window would ideally not grow it to fill the whole screen, but only the monitor it is shown on.

https://www.x.org/releases/X11R7.5/doc/man/man3/Xinerama.3.html