X11: Fix BadMatch on XSetInputFocus

Input focus was in some cases set before the window was visible (ICCCM
section 4.2.2).

Related to #798.
Fixes #789.

README.md

@@ -114,6 +114,8 @@ information on what to include when reporting a bug.
                    removed (#817,#826)
  - [X11] Bugfix: Window size limits were ignored if the minimum or maximum size
                  was set to `GLFW_DONT_CARE` (#805)
+ - [X11] Bugfix: Input focus was set before window was visible, causing
+                 `BadMatch` on some non-reparenting WMs (#789,#798)
  - [WGL] Added reporting of errors from `WGL_ARB_create_context` extension
  - [GLX] Bugfix: Dynamically loaded entry points were not verified
  - [EGL] Added `lib` prefix matching between EGL and OpenGL ES library binaries
@@ -180,6 +182,7 @@ skills.
  - Warren Hu
  - IntellectualKitty
  - Aaron Jacobs
+ - Erik S. V. Jansson
  - Toni Jovanoski
  - Arseny Kapoulkine
  - Osman Keskin

src/x11_window.c

@@ -92,6 +92,26 @@ static GLFWbool waitForEvent(double* timeout)
     }
 }
 
+// Waits until a VisibilityNotify event arrives for the specified window or the
+// timeout period elapses (ICCCM section 4.2.2)
+//
+static GLFWbool waitForVisibilityNotify(_GLFWwindow* window)
+{
+    XEvent dummy;
+    double timeout = 0.1;
+
+    while (!XCheckTypedWindowEvent(_glfw.x11.display,
+                                   window->x11.handle,
+                                   VisibilityNotify,
+                                   &dummy))
+    {
+        if (!waitForEvent(&timeout))
+            return GLFW_FALSE;
+    }
+
+    return GLFW_TRUE;
+}
+
 // Returns whether the window is iconified
 //
 static int getWindowState(_GLFWwindow* window)
@@ -1878,7 +1898,10 @@ void _glfwPlatformRestoreWindow(_GLFWwindow* window)
     }
 
     if (_glfwPlatformWindowIconified(window))
+    {
         XMapWindow(_glfw.x11.display, window->x11.handle);
+        waitForVisibilityNotify(window);
+    }
     else if (_glfwPlatformWindowVisible(window))
     {
         if (_glfw.x11.NET_WM_STATE &&
@@ -1915,8 +1938,11 @@ void _glfwPlatformMaximizeWindow(_GLFWwindow* window)
 
 void _glfwPlatformShowWindow(_GLFWwindow* window)
 {
+    if (_glfwPlatformWindowVisible(window))
+        return;
+
     XMapWindow(_glfw.x11.display, window->x11.handle);
-    XFlush(_glfw.x11.display);
+    waitForVisibilityNotify(window);
 }
 
 void _glfwPlatformHideWindow(_GLFWwindow* window)
@@ -1971,6 +1997,7 @@ void _glfwPlatformSetWindowMonitor(_GLFWwindow* window,
     if (window->monitor)
     {
         XMapRaised(_glfw.x11.display, window->x11.handle);
+        if (waitForVisibilityNotify(window))
             acquireMonitor(window);
     }
     else