wayland: Clarify the popup position restriction purpose

Child popups don't necessarily need to be within, but must intersect or be partially adjacent to the parent, per the xdg-positioner spec.

src/video/wayland/SDL_waylandwindow.c

@@ -242,12 +242,14 @@ static void ConfigureWindowGeometry(SDL_Window *window)
     SDL_SendWindowEvent(window, SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED, data->drawable_width, data->drawable_height);
 }
 
-static void EnsurePopupIsWithinParent(SDL_Window *window)
+static void EnsurePopupPositionIsValid(SDL_Window *window)
 {
-    /* Per the spec, popup windows *must* overlap the parent window.
-     * Failure to do so on a compositor that enforces this restriction
-     * will result in the window being spuriously closed at best, and
-     * a protocol violation at worst.
+    /* Per the xdg-positioner spec, child popup windows must intersect or at
+     * least be partially adjacent to the parent window.
+     *
+     * Failure to ensure this on a compositor that enforces this restriction
+     * can result in behavior ranging from the window being spuriously closed
+     * to a protocol violation.
      */
     if (window->x + window->w < 0) {
         window->x = -window->w;
@@ -1950,7 +1952,7 @@ int Wayland_CreateWindow(_THIS, SDL_Window *window)
     }
 
     if (SDL_WINDOW_IS_POPUP(window)) {
-        EnsurePopupIsWithinParent(window);
+        EnsurePopupPositionIsValid(window);
     }
 
     data->waylandData = c;
@@ -2088,7 +2090,7 @@ void Wayland_SetWindowPosition(_THIS, SDL_Window *window)
         xdg_popup_get_version(wind->shell_surface.xdg.roleobj.popup.popup) >= XDG_POPUP_REPOSITION_SINCE_VERSION) {
         int x, y;
 
-        EnsurePopupIsWithinParent(window);
+        EnsurePopupPositionIsValid(window);
         GetPopupPosition(window, window->x, window->y, &x, &y);
         xdg_positioner_set_offset(wind->shell_surface.xdg.roleobj.popup.positioner, x, y);
         xdg_popup_reposition(wind->shell_surface.xdg.roleobj.popup.popup,