Overlay scrollbars of unfocused windows may be hidden by other windows

Reproduce as follows:

1. Open an Evince window with a document open that requires scrolling, maximize it to the left half of the screen.
2. Maximize any other window to the right screen half, and make sure it is focused.
3. Move the cursor to the scrollbar area of the Evince window without changing the window focus.
4. See that the overlay scrollbar appears underneath the window on the right half.

I guess the overlay scrollbar popup should be always-on-top to prevent such situations.