I used to be able to go to the appearance tab or dungeon journal and CTRL click a piece of gear to bring up a preview, and it would not close the appearance or dungeon journal. Now the transmog window pops up closing the appearance tab and I have to scroll back through 20 pages of boots to go back to where I was looking.
So it isn’t working this way for you?
The above video was shot with a default game installation so as to ensure no addons were involved.
If it is not working that way for you then you might need to see if an addon could be interfering.
You might also need to do a full UI reset (as detailed in Support article linked below) to rule out a misbehaving addon or corrupted UI.
Your display isn’t big enough to see it all together with the new pane in the Dressing Room that shows all the names of the gear.
To fix it, click the icon in the top right corner of the Dressing Room that looks like a white scroll. It says “Appearance List” when you mouseover it. After you close the Appearance List, it will stay closed by default until you click it to display it again.
You have to do this on each separate toon. Once you do it on a toon, the preference for it to stay closed will persist through logouts.
If you could have your Collections tab and the Dressing Room open at the same time before 9.1.5, once you close the Appearance List you will be able to do it again. (Or dungeon journal, AH, etc.)