>The fact is in our design the input field and the panel are a single thing visually, and UX thinks that's important, thus we don't want to break it.
Yes, that's sensible. That is also in fact what Safari tries to do in most circumstances (by default the panel follows the size of the URLbar exactly). But if there's really not enough room because the URLbar is small, it will prefer the usability of enlarging the size over keeping the consistency. There's only so much you can do in a tiny panel.
Bug 1601330 Comment 3 Edit History
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>The fact is in our design the input field and the panel are a single thing visually, and UX thinks that's important, thus we don't want to break it.
Yes, that's sensible. That is also in fact what Safari tries to do in most circumstances (by default the panel follows the size of the URLbar exactly). But if there's really not enough room because the URLbar is small, it will prefer the usability of enlarging the size over keeping the consistency. There's only so much you can do in a tiny panel, even with clever tricks.