I'm not sure what you are doing, but I asked for `/tmp/something/session.conf` "after each time you close Geany in the steps above. Use another editor to look at the file, want Geany closed.". There are only three "close" instructions in that list but you have pasted five sets of identical values.
What I get at the three points where Geany is closed are:
``` treeview_position=156 geometry=0;0;2560;1388;1; ```
``` treeview_position=912 geometry=0;0;2560;1388;1; ```
``` treeview_position=912 geometry=320;0;1920;1388;0; ```
Note the change in `treeview_position` when the sidebar is resized and the change in geometry when Geany is unmaximised. These are expected.
Also I notice you have provided the value of "fullscreen". Full screen is _NOT_ maximised, maximised is the window control not `View->Fullscreen` (assuming whatever window manager you are using on Ubuntu has a maximise control). If you mean fullscreen that is a different thing.