rectOn
Where this placement puts the overlay on frame, as a fraction of it.
The point OverlayPlacement.overlayAnchor names inside the overlay lands on the point OverlayPlacement.frameAnchor names inside the frame, and OverlayPlacement.size gives the rectangle its extent. An overlay hanging over an edge answers a rectangle reaching outside 0f..1f, which is what a caller drawing the overlay's outline needs to see.
Return
The rectangle the overlay covers, in the frame's normalised space.
Parameters
frame
The frame the overlay is drawn on, in pixels.