Created attachment 308317 [details]
Screenshot, from the Widget Factory. See the “chaotic” / “Otto” line.

Using Adwaita, the selected row of a backdrop GtkTreeView is always blue, but there’s a difference with when the window is on top: the inside decorations (radiobuttons, checkboxes…) are drawn flat instead of with relief.

When you use the HighContrast theme, decoration are always flat, and the selected row is always drawn black (with white text). I think it should be grey when in backdrop, as are toggled buttons, text selections, etc.

“Funny thing” (for not calling it a bug), when you change from the inspector from Adwaita to HighContrast, the selected row is of the color I propose.