In particular: that commit removed a `transform` decl on `dialog:-moz-modal-dialog {`. which maybe we were accidentally relying on to make the dialog a container for fixed-pos elements...
Bug 1799036 Comment 5 Edit History
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In particular: that commit removed a `transform` decl on `dialog:-moz-modal-dialog`, which maybe we were accidentally relying on to make the dialog a container for fixed-pos elements...