Hello there.
So I’m still confused, this bug apeard on the first November, when i updated my Glyphs from… 3151 to 3260. Now I’ve downloaded the 3337.
Well now I think it’s not a bug, perhaps it’s because my OSX is the ELCAPITAN (10.11.6).
Anyway it’s still very confusing. Let’s take a look here.
Can anybody help me with? Any idea or suggestion?
Does that only happen in edit view? Or is it the same if you try in font view?
It works in the font window! I can overwrite the metrics… 
(that’s already something :))
But in the edit view looks like the LSB and RSB are locked. What is still annoying when you try to work on the sidebearings.
Sorry that it took so long. But I fixed it. The Problem was only with macOS 10.11 (Is it really a good idea to still use such an old system. I needed to jump several hoops to download the latest version on the test machine.)
The update is a cutting edge version (for now). Here is a direct download link: https://updates.glyphsapp.com/Glyphs3.3.1-3338.zip
That’s awesome. Thank you very much!