I didn't try it *all*, but IIUC the behavior isn't that complex: if the tab is already active, the click focuses the tab's label, otherwise it tries not to move focus and only switch page, focusing whatever was in that tab.
On one hand, I agree that there don't seem to me much use for mouse-focusing the tab label itself for regular tab labels (e.g. ones that don't have interactive elements in them). OTOH, I don't really see in which workflow it's an issue: you don't need clicking the tab again, and if you're focusing the tab with the mouse anyway, won't you use the mouse to interact with the tab's content as well, not caring where the focus actually is 99% of the time? Also note that the tab label also gets focused if there isn't anything focusable in the tab itself. It's unclear where else the focus could land in that case.
Also, there's a visual indication for me (using Adwaita): the tab label has the usual focus marker of a dashed outline.
Finally, note that in the case of Geany itself, you can assign a shortcut for focusing the sidebar, which could potentially help (in case the mouse click is a workaround). It doesn't magically move the focus out of the tab label, but it avoids accidentally clicking the tab and moving the focus.