GtkGridView should let you start a rubberband past the last row

Steps to reproduce

1. Run "Colors" from gtk4-demo
2. Change from showing 4096 colors to just 8 in the header
3. Try to start a rubberband selection from say, the middle of the GtkGridView and drag up to select some of those 8 colors

Current behavior

No rubberband selection is started and the console print a warning: Could not start rubberbanding: No item

Expected outcome

I would expect a rubberband to be able to be started in this empty space, much like you can in the empty space to the right of the color row.

Version information

gtk4-demo --version reports 3.99.5, but I'm using the nightly GNOME sdk flatpak, so I'm not sure exactly.