Often (but not all the  time), when I press F12 and HTML pane is in autohide, it often briefly flashes and hides again. I have to press F12 at least 2 more times to have it stay in position.

 

I think it happens mostly when coming back from another app, with alt-tab. E.g. : Alt-Tab to something else, select an item in IQ and press F12.

 

(Maybe will this somewhat address the perceived HTML pane sluggishness evoked in First post - comments and questions.)