From my code analysis, this is related to bug 1815607. Restoring `class="menuitem-iconic"` aligns the checkmark icon correctly when it's there, but now misaligned without the checkmark. Btw, all of `menuitem type=checkmark` suffer from the same slight misalignment, e.g. `Mark > Starred` from msg context menu. Firefox doesn't seem to have this problem, e.g. for `File > Work offline`.
Bug 1831159 Comment 1 Edit History
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From my code analysis, this is related to bug 1815607. Restoring `class="menuitem-iconic"` aligns the checkmark icon correctly when it's there, but now misaligned without the checkmark. Btw, all of `menuitem type="checkbox"` suffer from the same slight misalignment, e.g. `Mark > Starred` from msg context menu. Firefox doesn't seem to have this problem, e.g. for `File > Work offline`.