Window titlebars are triangle-shaped in flat design on Firefox

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(This might be a browser bug, I don't know. I couldn't get it to happen on Chrome.)
User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/115.0 (Windows 10, Firefox 115.0b9)
Snap! version: 8.2.3

Fine for me flat design FF Windows11
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it doesn't happen on mac.
maybe its just firefox

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This issues seems to have been fixed for me, might have just been a weird Firefox bug.