I get this message at the bottom of the page of the control panel: "You've gone idle, please enter your password to continue."
I just updated to ExpressionEngine to 2.7.3
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To me that looks like a JS conflict with a cp add-on like Devot:EE monitor or a straight out EE theme error. Presumably on a new entry screen you'll see the file manager modal down there too.
Try replacing/re-uploading the themes folder for EE, updating the accessories to latest (or any extensions which modify the CP JS), then clear your browser cache.