ewmh: Don’t push workspaces with width=0 and height=0 (Thanks Atsutane, badboy)

Even though i3 cannot know the width/height of some workspaces as
long as they are not initialized (say you used workspace 1 and 3,
but not workspace 2), some applications require this information.
In this case, it was Firefox which intersects the available workareas
(see mozilla/gfx/src/gtk/nsScreenGtk.cpp) and did not position some
windows correctly when being confronted with zero-width/height
workspaces.

src/ewmh.c

@@ -58,14 +58,41 @@ void ewmh_update_active_window(xcb_window_t window) {
 void ewmh_update_workarea() {
         Workspace *ws;
         int num_workspaces = 0, count = 0;
+        Rect last_rect = {0, 0, 0, 0};
+
         /* Get the number of workspaces */
-        TAILQ_FOREACH(ws, workspaces, workspaces)
+        TAILQ_FOREACH(ws, workspaces, workspaces) {
+                /* Check if we need to initialize last_rect. The case that the
+                 * first workspace is all-zero may happen when the user
+                 * assigned workspace 2 for his first screen, for example. Thus
+                 * we need an initialized last_rect in the very first run of
+                 * the following loop. */
+                if (last_rect.width == 0 && last_rect.height == 0 &&
+                    ws->rect.width != 0 && ws->rect.height != 0) {
+                        memcpy(&last_rect, &(ws->rect), sizeof(Rect));
+                }
                 num_workspaces++;
+        }
+
         DLOG("Got %d workspaces\n", num_workspaces);
         uint8_t *workarea = smalloc(sizeof(Rect) * num_workspaces);
         TAILQ_FOREACH(ws, workspaces, workspaces) {
-                DLOG("storing %d: %dx%d with %d x %d\n", count, ws->rect.x, ws->rect.y, ws->rect.width, ws->rect.height);
+                DLOG("storing %d: %dx%d with %d x %d\n", count, ws->rect.x,
+                     ws->rect.y, ws->rect.width, ws->rect.height);
+                /* If a workspace is not yet initialized and thus its
+                 * dimensions are zero, we will instead put the dimensions
+                 * of the last workspace in the list. For example firefox
+                 * intersects all workspaces and does not cope so well with
+                 * an all-zero workspace. */
+                if (ws->rect.width == 0 || ws->rect.height == 0) {
+                        DLOG("re-using last_rect (%dx%d, %d, %d)\n",
+                             last_rect.x, last_rect.y, last_rect.width,
+                             last_rect.height);
+                        memcpy(workarea + (sizeof(Rect) * count++), &last_rect, sizeof(Rect));
+                        continue;
+                }
                 memcpy(workarea + (sizeof(Rect) * count++), &(ws->rect), sizeof(Rect));
+                memcpy(&last_rect, &(ws->rect), sizeof(Rect));
         }
         xcb_change_property(global_conn, XCB_PROP_MODE_REPLACE, root,
                             atoms[_NET_WORKAREA], CARDINAL, 32,