Since moving to px for padding, the button height is determined by a mixture of ems and px, whereas the toolbar height is just in ems. This means when the font size changes the heights don't match up.
OOUI demos:
VE:
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Event Timeline
Restricted Application added a subscriber: Aklapper. · View Herald TranscriptOct 4 2021, 11:02 AM2021-10-04 11:02:11 (UTC+0)
matmarex closed this task as a duplicate of T291142: Gap between OOUI ProcessDialog primary action button and header bottom.Oct 4 2021, 1:20 PM2021-10-04 13:20:42 (UTC+0)