It would be nice to have a setting to configure the tab alignment/width in Builder (halign property), for instance, GTK_ALIGN_START:
Most other GUI editors have this setting built in, and I am personally very used to having tabs aligned to the start as seen above, and little issues and inconsistencies like this drive me crazy. Just some small tweaks like this would help a lot with staying comfortable in an IDE where you typically spend hours a day in.
This shouldn’t be too difficult to add, as GTK already gracefully handles different alignments like this and turns it scrollable when there are too many tabs. Another option to change the tab width along side this would also be nice.