Cursor position is incorrectly placed until first character is typed
When GtkTextTag:pixels-above-line
is set, the cursor might be in the wrong location until the first character is typed. Set it to something like 10 and then notice it move as soon as the first character is typed.
I do notice that this only happens when at the end of the buffer, so I might venture a guess that it is related to the GtkTextTag ending at the current cursor position (ie: toggle-off).
Probably just needs to be added to cursor rect calculation for empty line when toggle-off + end of buffer.