In r1182656 for T400953, I'm adding a Codex dialog that is displayed after edits (code). This works with the source editor, but not with VE. Upon closer inspection, I can see that the dialog is mounted, and displayed, but it is completely invisible due to the following CSS rule:

.cdx-dialog-backdrop {
    opacity: 0;
}

This is coming from codemirror.less. The rule is problematic because it styles elements it doesn't own, therefore breaking functionality for other Codex user on the same page. I still haven't looked into what the rule is trying to achieve, but if it is needed, I believe it should find a way to target the intended element only.