It seems that GNOME Shell under Wayland does not provide standard cursor feedback for mpv window edges.
More accurately, GNOME Shell doesn't provide "window decorations" to Wayland windows at all. The window in its entirety has to be drawn and managed by the program itself, and this includes defining "edges" and cursor feedback, the same way an app would change cursors when hovering over a text field or hyperlink.
Most programs either use libdecor or rely on their UI toolkit (such as GTK or Qt, or e.g. Chrome has its own thing) to do the job – for example, the edges and even shadows of a GTK4 window are defined as part of the overall GTK4 styling.
Mpv naturally doesn't want to use either of the above.
Is this a known limitation of GNOME Shell under Wayland?
Yes. It's widely known that while various other Wayland compositors implement a protocol to request "server-side window decorations" (which was the default mode in X11), GNOME doesn't do that and insists that Wayland apps draw their own, e.g. via libdecor.
However, this was originally part of Wayland design – all apps were expected to handle window decorations by themselves. Over time, other compositors decided to add back an option for X11-style server-side decorations, while GNOME stuck to their original plan.
So it's more precisely a limitation by design, not a known bug or such.
Are there any workarounds to get cursor feedback near mpv window edges?
Use Alt+drag or Super+drag to move and resize the window. (I think the default is Alt but I always change it to Super (the Windows key); you can set this in gnome-tweaks.)
When you Alt+right-click drag to resize the window, the mouse merely needs to be in the same quadrant and not necessarily on the actual edge, so cursor feedback becomes unnecessary.
Use celluloid (aka gmpv), which is a GTK4 frontend around mpv and accordingly has "proper" GTK-styled window borders. (And also playback controls which don't shrink to microscopic size like in mpv.)
(If you're not using mpv-specific features, then GStreamer-based GTK4 video players such as clapper or showtime might be acceptable.)
