osc: fix hovering timestamp sticking around when moving mouse away

The OSC calls this "tooltip" (and although a general mechanism, there's
only one instance using it). One particular problem was that with the
default OSC layout, moving the mouse down and out of the window, the
tooltip stuck around, because the returned mouse position was the last
pixel row in the window, which still overlaps with the seek bar.

Instead of introducing mouse_in_window, you could check last_mouse_X for
nil, but I think this is clearer.

This returns (-1, -1) to the caller if the mouse is outside. Kind of
random, but works.

player/lua/osc.lua

@@ -102,6 +102,7 @@ local state = {
     mp_screen_sizeX, mp_screen_sizeY,       -- last screen-resolution, to detect resolution changes to issue reINITs
     initREQ = false,                        -- is a re-init request pending?
     last_mouseX, last_mouseY,               -- last mouse position, to detect significant mouse movement
+    mouse_in_window = false,
     message_text,
     message_hide_timer,
     fullscreen = false,
@@ -160,9 +161,13 @@ end
 
 -- return mouse position in virtual ASS coordinates (playresx/y)
 function get_virt_mouse_pos()
-    local sx, sy = get_virt_scale_factor()
-    local x, y = mp.get_mouse_pos()
-    return x * sx, y * sy
+    if state.mouse_in_window then
+        local sx, sy = get_virt_scale_factor()
+        local x, y = mp.get_mouse_pos()
+        return x * sx, y * sy
+    else
+        return -1, -1
+    end
 end
 
 function set_virt_mouse_area(x0, y0, x1, y1, name)
@@ -2212,6 +2217,7 @@ function mouse_leave()
     end
     -- reset mouse position
     state.last_mouseX, state.last_mouseY = nil, nil
+    state.mouse_in_window = false
 end
 
 function request_init()
@@ -2449,6 +2455,8 @@ function process_event(source, what)
 
     elseif source == "mouse_move" then
 
+        state.mouse_in_window = true
+
         local mouseX, mouseY = get_virt_mouse_pos()
         if (user_opts.minmousemove == 0) or
             (not ((state.last_mouseX == nil) or (state.last_mouseY == nil)) and