The same for me, both resize and pbm_size events are fired.
newheight argument is 17 pixels less when horizontal scrollbar appears, but it could be different depending on windows theme.
If your Pb works like mine, sometimes breakpoints just don't break, it may be an explanation.
You could also have the y of your text object based on describe('datawindow.horizontalscrollmaximum'), but i think it's a worse solution.