I can reproduce this bug in Nightly 2020-09-02 with the site linked in comment 0, but I can't reproduce in current Nightly, so I think was fixed at some point. The fix range for me on Linux is: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=93831b5983e970149c230c9508d72656f7a704d0&tochange=aef48291d22e9a2b8c579e19113e7ab195a91ed7 In there, this would've been " Bug 1689359 - Enable non-native-theme on Linux Nightly by default." So this was essentially fixed by our non-native theme work, which was under metabug 1535761. Resolving as FIXED with a dependency on that bug.
Bug 1662832 Comment 3 Edit History
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I can reproduce this bug in Nightly 2020-09-02 with the site linked in comment 0, but I can't reproduce in current Nightly, so I think was fixed at some point. The fix range for me on Linux is: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=93831b5983e970149c230c9508d72656f7a704d0&tochange=aef48291d22e9a2b8c579e19113e7ab195a91ed7 In there, this would've been " Bug 1689359 - Enable non-native-theme on Linux Nightly by default." So this was essentially fixed by our non-native theme work, which was under meta-bug 1535761. Resolving as FIXED with a dependency on that bug.