Well, I meant 100% reproducible for ME... 
The demo version works correctly, although there are a couple of differences. One is that a new event opens in a 'shrunken' mode, with a red pulldown arrow below "no repeat / Change", and the box re-sizes correctly when you click the arrow.
The other more interesting difference is that the edit box for notification recipients has the "select recipients from list" emoji on the far RHS of the line, but in my instance it's on the next line, crucially making the form that much taller and forcing the scroll bar.
I've updated the image I linked to above with a blue arrow pointing it out.
Where's yours? In Firefox, mine is on the RHS, the same as your demo cal.
I tried making the box wider so the emoji returned to its 'correct' location, but when I closed and launched a new event the change in size wasn't retained, so I was back to the earlier problem.
But why's it only happening to me? Where do I (presumably) have a setting that's limiting the width and forcing this problem?
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HAHAHA! I've fixed it!
I'm running the calendar tab @ 110% zoom. If I go back to 100% everything's correct - just a little shrunken and my aged eyes disapprove. (If I go to 110% on the demo cal, my problem manifests there too).
Sorry to bother you with this one. An amusing little learning lesson.
- Greig.
