The modern replacement for a beloved dead tool
OnTopReplica alternative
OnTopReplica was the tool for floating a live clone of a window — and it hasn't seen a release in roughly a decade. It often still half-works on modern Windows, until it doesn't: DPI scaling, multi-monitor setups and newer apps all fight it. If you're searching for an alternative, you already know.
InfinitePIP is the modern take on the same idea, built on current Windows desktop capture: clone any window — or just a region of it — into a borderless, always-on-top panel. The region picker covers OnTopReplica's signature trick (watching one corner of a window), and it goes further: multiple simultaneous PiPs, full-screen and monitor capture, zoom, opacity, and virtual displays to park source windows on.
Free — pay what you want. Actively maintained, which is rather the point.