One window

Double-clicking a second image does not open a second viewer. The launch finds the one already running, hands its file over and exits, and the picture appears in the time it takes to decode — measured at 135 to 155 milliseconds against 320 to 560 for a cold start, because the window and the graphics device are already there.

The same answers multi-select. Windows starts one process per selected file, so five files means five launches; four of them hand their file to the first, and the five arrive as one list in one window. Arrow keys then walk that selection rather than the whole folder — the five you picked, not the hundreds beside them. Five files opened this way took 239 milliseconds altogether.

The window also comes forward when it takes a file. Windows only lets the process the user is working in raise a window, and after a double-click that process is the launch, not the viewer already running — so the launch hands that right over as it connects, naming the window it found. Without it a default viewer would change its picture behind whatever you were looking at.

The window that owns the channel is simply the first one to create it, which is a single atomic call, so two launches racing cannot both decide they are the window. Nothing polls: a hand-over wakes the event loop the same way a finished decode does, and a still image still costs no wakeups at all. See ADR 0016.