When the caret is placed in virtual space and a letter is typed, Geany back-fills using spaces. When the indentation mode is TABs, tabs should be used.
For instance, set indentation mode to TABs and open a source-code file which uses tabs for indentation. Go the the end of the file. Use the enter key to add a few blank lines. Go to Document -> Strip trailing spaces. Use the right arrow key to move to a column which is several tab widths in from the left border. Type a letter.
Since the editor mode is TABs, Geany should have inserted tabs to the left of the letter just typed. Instead, spaces are inserted.
12:22:01: Geany INFO : Geany 1.24 (git >= 0c5442b), en_US.UTF-8
12:22:01: Geany INFO : GTK 2.24.10, GLib 2.32.3