theme: window prelight border corner radius mismatch
The gtk Adwaita updated the border radii to be larger all over the place, including the window decorations. Windows switching in the overview now exposes a problem where the uniform border radius for the focused thumbnail creates a hole for the headerbar.
Would be nice to match the outline somewhat for the common case (ue to varying size of the thumbnails we probably shouldn't do non-uniform radius, but let's try increasing it a tad).