Wayland: Fix fallback decoration menu placement

The fallback decorations would place the menu at the wrong position, by
not translating the last decoration surface position into toplevel
surface coordinates.

This also limits the menu to the caption area of the top decoration
surface, similar to how other toolkits work.

README.md

@@ -139,6 +139,7 @@ information on what to include when reporting a bug.
                      fallback decorations
  - [Wayland] Bugfix: Fallback decorations would report scroll events
  - [Wayland] Bugfix: Keyboard repeat events halted when any key is released (#2568)
+ - [Wayland] Bugfix: Fallback decorations would show menu at wrong position
  - [X11] Bugfix: Running without a WM could trigger an assert (#2593,#2601,#2631)
  - [Null] Added Vulkan 'window' surface creation via `VK_EXT_headless_surface`
  - [Null] Added EGL context creation on Mesa via `EGL_MESA_platform_surfaceless`

src/wl_window.c

@@ -405,13 +405,19 @@ static void handleFallbackDecorationButton(_GLFWwindow* window,
     }
     else if (button == BTN_RIGHT)
     {
-        if (window->wl.xdg.toplevel)
-        {
+        if (!window->wl.xdg.toplevel)
+            return;
+
+        if (window->wl.fallback.focus != window->wl.fallback.top.surface)
+            return;
+
+        if (ypos < GLFW_BORDER_SIZE)
+            return;
+
         xdg_toplevel_show_window_menu(window->wl.xdg.toplevel,
                                       _glfw.wl.seat, serial,
-                                          window->wl.cursorPosX,
-                                          window->wl.cursorPosY);
-        }
+                                      xpos,
+                                      ypos - GLFW_CAPTION_HEIGHT - GLFW_BORDER_SIZE);
     }
 }