Overlay scrollbars allow interaction behind a modal dialog

This is reproducible in many applications. I will step you through reproducing it in in gedit.

First, open gedit with a document that needs a scrollbar. Now, open the About dialog. Note that it is impossible to interact with the document while the About dialog is open. Using the mouse wheel over it does nothing. However, you CAN scroll using the overlay scrollbars.

This is inconsistent, and it could lead to unexpected behaviour in certain applications: when a dialog is modal, it usually has a good reason to be, if for nothing else but to make developers' lives easier as they don't need to worry about odd corner cases. Instead, the overlay scrollbars should be disabled in this situation.