When I use 1.3.8 the preview moves to the side of the screen opposite the selected glyphs. So if I select H on the left edge of the screen, my preview appears near the right edge of the screen.
can you send me a screenshot?
Now working fine 
could be that fail erraticaly?
Glyphs 3.0.2 [3044] Selected glyph isn’t centered on preview.