@ralf3u That is my PPA. It was last built on 2021-11-30, and has several [patches](is:open is:pr author:xiota) that have not been merged (probably based on d8f2f14a16fe57fb1a98e767a2263846a0135431). (A one-line description of each patch is noted in the deb changelog.) That is the same build I have been using, and I have not personally had any problems with it.
One of the patches is #2979, which changes save/restore of geometry and position. However, I am unable to reproduce your issue with Kubuntu or XFCE. Since you have two computers behaving differently, there is probably something different between the two that is causing your issue.
What window manager are you using on each computer?
Do you still this problem when you disable all Geany plugins?
Do you still have this problem when you start Geany with a new config?
<pre>geometry=-1;-1;-1;-1;1;</pre>
Those values represent xpos; ypos; width; size; maximized. `-1;-1;-1;-1;0` are the default values when a new config is created. Geany normally rewrites the default values when the config is saved. My patch leaves some of the values alone when "Save window size" or "Save window position" are disabled.
In Edit / Preferences / General / Startup / Shutdown, what are the states of the checkboxes for "Save window size" and "Save window position"? (If either is unchecked, then sidebar position might be *expected* to *not* be saved.)
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When I run the elextr's scenarios, the behavior is as described, but the specific saved values are sometimes different than with unpatched Geany. (Unpatched Geany saves maximized window sizes when the window is maximized. My patch always saves the unmaximized size.)
1. start Geany, starts with the default size, not maximised 2. maximise geany, close... <pre>treeview_position=165<br/>msgwindow_position=399<br/>geometry=350;150;900;600;1;</pre> and reopen, sidebar has same width
3. change sidebar width, close... <pre>treeview_position=336<br/>msgwindow_position=399<br/>geometry=350;150;900;600;1;</pre> and reopen, sidebar retains new width
4. unmaximise geany... sidebar is the same width in pixels not percent. 5. close... <pre>treeview_position=336<br/>msgwindow_position=399<br/>geometry=350;150;900;600;0;</pre> and reopen, sidebar retains the same width