devtrackers.gg
When changing gear appearance to specific gear items such as: The Unfliching Vest, the Aggressor Heartbreaker Kneepads, Dosoftei Braces Kneepads, Umbra Initiative Kneepads and Mask certain the entire transmog and modification hud shifts off screen and can't be navigated at all. All you can do is use tab to strip the item.
I don't have all appearances unlocked yet so there are very likely more affected. Indentifying them is quite simple though, since before selecting them in the transmog menu the hud already shifts off screen as soon as the gear item loads.