Preconditions:
- Linux
- Chrome Dev 70.0.3538.9
- Have a card saved (uploaded) to Google Pay
- Using GTK+ theme (check chrome://settings/appearance under "Themes")

On any form with a credit card field, select the card from Google Pay. A dialog asking for a CVC will appear; fill one in and submit. The dialog should change to show a spinner and a waiting message. The background behind this spinner/message was a big grey rectangle instead of the white background used in the rest of the dialog. (This is consistent with other problems we've seen with grey backgrounds on GTK+).

Unfortunately this happened on my personal account (I was actually making a purchase!) and went away before I could get a screenshot.