firefox - maximize it, open a long page, push the mouse to the right edge, press left mouse button, drag the mouse down - the scrolling works. Try the same with geany - and you cannot just push the mouse cursor to the right side, click and scroll
So its not that Geany doesn't go fullscreen?
The difference between Geany and friedfox is that in Geany the edit widget with scrollbar is in a horizontal pane and your GTK theme has put a very small but not zero width border on the pane. The same happens on the left, a mouse pushed right to the edge won't select a line in the symbols but move it in a few pixels and its fine.