Firefox Mobile + Gboard behaves poorly

I reviewed the state of the latest CodeMirror on various Android browsers and keyboards. I found that my personal favorite combination, Chrome + SwiftKey, now works very well, so that's what I'll recommend to my users. I found some problems with Firefox and Gboard individually, and especially together. Here's one consistent reproduction sequence that might be helpful for debugging:

• Load https://codemirror.net/demo/sublime.html
• Tap right after a comma at the end of one of the lines (e.g. line 2)
• Swipe a word like "hello" (without lifting your finger)
• The cursor ends up to the left of "hello", instead of to the right of "hello" as it should.
• Swiping another word like "world" puts that word before "hello" instead of "after"