Commands: top/bottom looks cut.

build (&general): left part is to wide and stays wide on cost of right. Is different from before.

That's what I see:

What style settings do you use? Which Qt? For reference: TeXstudio 4.4.2 (git 4.4.1-3-ga9d878dca), Using Qt Version 6.4.1, compiled with Qt 6.4.1 R.

The width of the select page part is the same as I see from new installed 4.4.1 Win10. Be aware that there is currently no splitter (is expected to be done with the other issue). Reference (old) images show current state of 4.4.1, so no difference I can see.

Here 4.4.2 (mine) is on top of 4.4.1: Only difference is the smaller gap between page list and page, so page list is wider:

Maybe I can figure out which value influences the width of page list.

have you tried to resize the dialog and see what happens ?

yes, but no bad observations. Can it be that a cpp program defines size of the dialog and of the page list?

Following image shows the width when the list part starts shrinking on further reduction of the dialog width:

This new version can reach a smaller width because the part right to the list is smaller (less spacing). In fact the old space in the middle consists of two column separators of width 18px each. This is possible because there is a second but empty column of width 0 (well it contains a vertical spacer of width 0 otherwise this would not be possible). I removed that empty column (and thus one column separator) and reduced the width for column separators to 12px. This is the new space in the middle.

your image above shows the issue. right pane is reduced in size, starting with scrolling, while the left pane is still at max width.
You can set scaling ratios between different elements and my guess is that you removed the definition for the right-hand side.
This all formular stuff, no cpp involved.

I reworked my first post incl. images and finished work.

looks good.
Thanks.