X11: Fixed XSetInputFocus before VisibilityNotify

Should resolve glfw/glfw#789 issue, where calls to glfwCreateWindow
might fail on non-reparenting window managers (e.g. dwm and xmonad)
since they don't set their visibility flag, right after XMapWindow.
Attempting to print 'map_state' right before XSetInputFocus results
either in IsUnmapped or IsUnviewable, on the tested WM (dwm 6.1-3).
Which gives BadMatch errors on XSetInputFocus as shown in Xlib doc.
Hasn't been tested on any additional X11 window managers as of yet.

README.md

@@ -73,6 +73,7 @@ does not find Doxygen, the documentation will not be generated.
 
 ## Changelog
 
+ - [X11] Bugfix: Didn't wait until window was visible before setting focus (#789)
  - Bugfix: Single compilation unit builds failed due to naming conflicts (#783)
  - Bugfix: The range checks for `glfwSetCursorPos` used the wrong minimum (#773)
  - [Win32] Bugfix: `glfwSetClipboardString` created an unneccessary intermediate

src/x11_platform.h

@@ -112,6 +112,7 @@ typedef struct _GLFWwindowX11
     XIC             ic;
 
     GLFWbool        overrideRedirect;
+    GLFWbool        visuallyMapped;
 
     // Cached position and size used to filter out duplicate events
     int             width, height;

src/x11_window.c

@@ -604,6 +604,7 @@ static GLFWbool createWindow(_GLFWwindow* window,
 
     _glfwPlatformGetWindowPos(window, &window->x11.xpos, &window->x11.ypos);
     _glfwPlatformGetWindowSize(window, &window->x11.width, &window->x11.height);
+    window->x11.visuallyMapped = GLFW_FALSE; // Assume window isn't mapped yet.
 
     return GLFW_TRUE;
 }
@@ -1365,6 +1366,29 @@ static void processEvent(XEvent *event)
             return;
         }
 
+        case VisibilityNotify:
+        {
+            int visibilityState = event->xvisibility.state;
+            // Some window managers (mostly non-reparenting
+            // like dwm, xmonad, ratpoision, openbox, cwm),
+            // don't set the visibility flag after mapping,
+            // a pre-condition for e.g. XSetInputFocus()...
+            // leads to BadMatch error as described in X11.
+            // Most of these will however notify with event
+            // VisibilityFullyObscured on visibility unset.
+            if (visibilityState != VisibilityFullyObscured)
+                window->x11.visuallyMapped = GLFW_TRUE;
+            return;
+        }
+
+        case UnmapNotify:
+        {
+            // Should be the only case when a window
+            // becomes completely visually unmapped.
+            window->x11.visuallyMapped = GLFW_FALSE;
+            return;
+        }
+
         case FocusIn:
         {
             if (window->cursorMode == GLFW_CURSOR_DISABLED)
@@ -1925,6 +1949,11 @@ void _glfwPlatformFocusWindow(_GLFWwindow* window)
     else
     {
         XRaiseWindow(_glfw.x11.display, window->x11.handle);
+        // Requested window might not have been completely initialized
+        // by the window manager at this point, need to wait until the
+        // window has both been mapped and also set a visibility flag.
+        // Wait for VisibilityNotify X11 event before setting a focus.
+        while (!window->x11.visuallyMapped) _glfwPlatformWaitEvents();
         XSetInputFocus(_glfw.x11.display, window->x11.handle,
                        RevertToParent, CurrentTime);
     }