I think this is because it cannot shrink anymore due to the controls above the tree view in the Files tab.
Yes me too, when i toggle on/off the 'File' plugin, this issue becomes more/less.
Thanks for all your comment, at least it becomes clear, that we cant do something right now :) ( ... maybe if i dont have to install much to build with gtk2 ... ? ) Anyway,
since i'm not developer of geany, i'm not sure if i must close this or leave it for more comments, maybe a moderator here knows better ... ,
thank you all