Incorrect positioning of tooltips on vertical toolbars when going from one button to another
When you've got a vertical toolbar like in Pitivi, when moving from one button to another (instead of making the mouse exit and re-enter the toolbar), the tooltip positioning is incorrect and varies depending on the direction you're going in (upwards or downwards), often being cut off from the screen (and thus unreadable). This only happens on vertical toolbars. In this particular case, Pitivi is using the inline-toolbar style but it shouldn't affect things.
Video demonstrating the issue (affects all versions of GTK3 from 2012 until this day at least):