This issue occurs when two panels are open in a single tab (thus having the “double tab” appearance in the tab itself) and then clicking “download” on a pdf within the file field. The pdf viewer opens. I am then unable to close the tab (or either of the “sub tabs”). The “x” close buttons are unresponsive. I would expect this to still work.
Using the back button to back out of the pdf viewer still works, and the tabs are able to be closed then. Also, this issue does not occur if there was only one panel open before attempting to download the pdf.
This is quite minor as backing out is very easy. Just seemed like a bug, so I thought I’d report it.