Hello 
Maybe a silly question, but: I can’t figure out how to move the horizontal beams…
The same way as you move the built in measurement line in (T)ext mode. Hold down CTRL+OPT+CMD and Glyphs should show the built in line. Move it with your mouse, when you go back to your normal edit mode, the Show Stems (Light) line should be in the now location.
Does that help?
Yes, thanks! I didn’t realize that I had to go into text mode…
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