https://redd.it/tygm1s/

DllCall("User32\SetWinEventHook"
    , "Int",0x8004 ; EVENT_OBJECT_REORDER
    , "Int",0x8004
    , "Ptr",0
    , "Ptr",RegisterCallback("WindowResize", "F")
    , "Int",0
    , "Int",0
    , "Int",0)

return ; End of auto-execute

Blank(Set)
{
    if (Set) {
        Gui Blank:New, AlwaysOnTop -Caption -Border
        Gui Color, 0x000000
        Gui Show, NoActivate x1921 y0 w100 h100
    } else {
        Gui Blank:Destroy
    }
}

WindowResize(hWinEventHook, event, hWnd) ;, idObject, idChild, idEventThread, dwmsEventTime)
{
    static dimensions := ""

    WinGet exe, ProcessName, % "ahk_id" hWnd
    if (exe != "mpv.exe")
        return
    WinGetPos x, y, w, h, % "ahk_id" hWnd
    if (x y w h = dimensions)
        return
    dimensions := x y w h
    isFs := (dimensions = "0019201080")
    Blank(isFs)
}