So in "old builds" (from ~2011 through 2016), the select and the button are the same height, and the text-input element is a bit smaller (on its own). That kinda made sense, because select elements render like a button. But in December 2016, the button got a little smaller (becoming the same height as the text-input element) leaving `<select>` all on its own. The regression range for that change is: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=f65ad27efe839ce9df0283840a1a40b4bbc9ead0&tochange=557548714db55136b51e1129d649e2599797985f And I think the relevant change there was: > https://hg.mozilla.org/mozilla-central/rev/f86ce2e2f6a51f351974e3781158bc0586ab71ed > Thomas Wisniewski — Bug 140562 - Part 2: Remove the 2px extra padding on buttons for a prospective -moz-focus-inner ring, and just size that ring the same as the content frame (inflated by its CSS padding). r=dbaron I think that "2px extra padding" that we removed there is what we're trying to mimic with the select forms.css declarations that I linked in comment 0. We should probably just remove that from forms.css, now that the corresponding button padding is gone.
Bug 1560824 Comment 3 Edit History
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So in "old builds" (from ~2011 through 2016), the select and the button are the same height, and the text-input element is a bit smaller (on its own). That kinda made sense, because select elements render like a button. But in December 2016, the button got a little smaller (becoming the same height as the text-input element) leaving `<select>` all on its own. The commit range for that change is: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=f65ad27efe839ce9df0283840a1a40b4bbc9ead0&tochange=557548714db55136b51e1129d649e2599797985f And I think the relevant change there was: > https://hg.mozilla.org/mozilla-central/rev/f86ce2e2f6a51f351974e3781158bc0586ab71ed > Thomas Wisniewski — Bug 140562 - Part 2: Remove the 2px extra padding on buttons for a prospective -moz-focus-inner ring, and just size that ring the same as the content frame (inflated by its CSS padding). r=dbaron I think that "2px extra padding" that we removed there is what we're trying to mimic with the select forms.css declarations that I linked in comment 0. We should probably just remove that from forms.css, now that the corresponding button padding is gone.