I don't normally use the cover browser so didn't see the behavior changes.
In 7.3 on windows 10:
- If not in a separate window, in "narrow" view it opens to the right of the booklist. It used to open over the booklist, which is where I want it. I don't see a preference that controls this.
- Using in a separate window:
- If I use the cover browser button it opens and closes as expected.
- If I click on the X in the cover browser window title bar, it closes and immediately reopens over a now-empty window.
- If after clicking the X I click the cover browser button, the window doesn't close. However, now the X works.
- With the "real" cover browser open (using the button), when I close one of the empty windows with the X it either doesn't go away or reopens very quickly, now showing the cover browser. The window where it was shown is now empty.
- If calibre thinks the cover browser is closed, the X works on the empty windows.