Windows 7, Toad 18.104.22.168
While editing a 700-line package body, I began seeing a box at the bottom of the editor window. It started at about column 35 with size eight characters wide and two lines tall. It only flashed briefly when clicking anywhere in the window with the mouse, scrolling down (not up) with the mouse wheel, or using any of the keyboard cursor keys. It would flash with each action. Rapid actions would produce rapid flashing.
The package had a single bookmark (#1) at about line 500. I navigated to the bookmark several times during the session by pressing . I think what happened is that the box graphic that starts large and focuses in on the bookmarked line got stuck at the middle size and kept flashing as I moved about the code.
I haven’t been able to reproduce this behavior.