With bug 1656301, the API we use to draw the separator is causing artifacts on macOS 11. (It's a deprecated API). As a replacement, we are drawing the separator with a fixed color on macOS 11 only. It is difficult for me to judge the problem with the color. Can we live with this problem or do we need to adjust the color before shipping? There are some other known issues with the popup differing from the native macOS popups and we have bug 1668119 filed to address those. The code responsible for the color is here. https://searchfox.org/mozilla-central/rev/f21850ca45036ddb84cd25aa355a6969d7c94c4f/widget/cocoa/nsNativeThemeCocoa.mm#1216-1219
Bug 1656301 Comment 51 Edit History
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Moved my comment to bug 1668493 comment 3.