It seems like the redrawing in the completion entry popup window doesn't seem to consider state changes caused by scrolling.
To reproduce the problem:
- start gedit
- open
- go to your jhbuild checkout directory (or any other folder with a large enough number of files/folders)
- type g to start autocompletion (or any other character that results in a large enough list of suggestions to have a scrolled window)
- while the cursor is on some entry in the suggestion list start scrolling while not moving the cursor

-> This causes different entries to get highlighted in the suggestion list but they don't seem to get correctly redrawn once they are not highlighted anymore leaving them in a highlighted state or with other redraw issues.