Shift-Tab does different stuff in Geany. When cursor is within leading indentation, it subtracts a level; it does this correctly whether cursor starts at a tab stop or not. When cursor is somewhere after the first printable char, it just moves the cursor back to the previous stop; it doesn't alter the line at all.

But if Shift-Tab caused little green worms to crawl out of the window, I wouldn't use hard tabs anymore. Lesson learned.