XUL panel elements with default level can obscure panels with titlebar attribute of normal. This doesn't happen on OS X. To see an example, open the Inspector (from the Tools menu or Web Developer>Inspect with the pref devtools.inspector.enabled set to true in About:Config). Highlight a large element, eg, a page's <body> or a full-length <div>. See the yellow highlighter panel obscure the top part of the Inspector's tree panel (which has titlebar=normal). If a piece of the titlebar in the tree panel is showing, you can raise the tree panel over the highlighter.
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