X11: Fix window made non-floating by being hidden

The previous implementation was based on the incorrect assumption that
the _NET_WM_STATE_ABOVE state is always retained on unmapped windows.
According to EWMH the WM should remove the _NET_WM_STATE property when
a window is unmapped.

This commit moves the adding of _NET_WM_STATE_ABOVE to before mapping
the window during glfwShowWindow.  The logic for removing
_NET_WM_STATE_ABOVE from hidden windows is retained, as EWMH still
allows WMs to leave the window property on unmapped window.

Fixes #2276

README.md

@@ -153,6 +153,7 @@ information on what to include when reporting a bug.
  - [X11] Bugfix: Prevent BadWindow when creating small windows with a content scale
    less than 1 (#2754)
  - [X11] Bugfix: Clamp width and height to >= 1 to prevent BadValue error and app exit
+ - [X11] Bugfix: Floating windows silently became non-floating when hidden (#2276)
  - [Linux] Bugfix: The header for `ioctl` was only implicitly included (#2778)
  - [Null] Added Vulkan 'window' surface creation via `VK_EXT_headless_surface`
  - [Null] Added EGL context creation on Mesa via `EGL_MESA_platform_surfaceless`

src/x11_window.c

@@ -2438,6 +2438,38 @@ void _glfwShowWindowX11(_GLFWwindow* window)
     if (_glfwWindowVisibleX11(window))
         return;
 
+    if (window->floating && _glfw.x11.NET_WM_STATE && _glfw.x11.NET_WM_STATE_ABOVE)
+    {
+        Atom* states = NULL;
+        const unsigned long count =
+            _glfwGetWindowPropertyX11(window->x11.handle,
+                                      _glfw.x11.NET_WM_STATE,
+                                      XA_ATOM, (unsigned char**) &states);
+
+        // NOTE: We don't check for failure as this property may not exist yet
+        //       and that's fine (and we'll create it implicitly with append)
+
+        unsigned long i;
+
+        for (i = 0;  i < count;  i++)
+        {
+            if (states[i] == _glfw.x11.NET_WM_STATE_ABOVE)
+                break;
+        }
+
+        if (i == count)
+        {
+            XChangeProperty(_glfw.x11.display, window->x11.handle,
+                            _glfw.x11.NET_WM_STATE, XA_ATOM, 32,
+                            PropModeAppend,
+                            (unsigned char*) &_glfw.x11.NET_WM_STATE_ABOVE,
+                            1);
+        }
+
+        if (states)
+            XFree(states);
+    }
+
     XMapWindow(_glfw.x11.display, window->x11.handle);
     waitForVisibilityNotify(window);
 }
@@ -2667,6 +2699,10 @@ void _glfwSetWindowFloatingX11(_GLFWwindow* window, GLFWbool enabled)
     }
     else
     {
+        // NOTE: _NET_WM_STATE_ABOVE is added when the window is shown
+        if (enabled)
+            return;
+
         Atom* states = NULL;
         const unsigned long count =
             _glfwGetWindowPropertyX11(window->x11.handle,
@@ -2677,26 +2713,7 @@ void _glfwSetWindowFloatingX11(_GLFWwindow* window, GLFWbool enabled)
         // NOTE: We don't check for failure as this property may not exist yet
         //       and that's fine (and we'll create it implicitly with append)
 
-        if (enabled)
-        {
-            unsigned long i;
-
-            for (i = 0;  i < count;  i++)
-            {
-                if (states[i] == _glfw.x11.NET_WM_STATE_ABOVE)
-                    break;
-            }
-
-            if (i == count)
-            {
-                XChangeProperty(_glfw.x11.display, window->x11.handle,
-                                _glfw.x11.NET_WM_STATE, XA_ATOM, 32,
-                                PropModeAppend,
-                                (unsigned char*) &_glfw.x11.NET_WM_STATE_ABOVE,
-                                1);
-            }
-        }
-        else if (states)
+        if (states)
         {
             for (unsigned long i = 0;  i < count;  i++)
             {