Handle the EWMH atom _NET_WM_DESKTOP.

We already claim _NET_WM_DESKTOP support in _NET_SUPPORTED since around 2009,
but haven't actually done anything with it. However, especially pagers like
gnome-panel rely on this property to be updated and many tools, like GTK, want
to use the corresponding client messages to make a window sticky, move it
around etc.

This patch implements full support according to the EWMH spec. This means:

* We set the property on all windows when managing it.
* We keep the property updated on all windows at all times.
* We read and respect the property upon managing a window if it was set before
  mapping the window.
* We react to client messages for it.
* We remove the property on withdrawn windows.

Note that the special value 0xFFFFFFFF, according to the spec, means that the
window shall be shown on all workspaces. We do this by making it sticky and
float it. This shows it on all workspaces at least on the output it is on.

Furthermore, the spec gives us the freedom to ignore _NET_WM_DESKTOP when
managing a window if we have good reason to. In our case, we give window
swallowing a higher priority since the user would likely expect that and we
want to keep placeholder windows only around for as long as we have to.
However, we do prioritize this property over, for example, startup
notifications.

fixes #2153
fixes #1507
fixes #938

src/ewmh.c

@@ -21,24 +21,9 @@ xcb_window_t ewmh_window;
  *
  */
 void ewmh_update_current_desktop(void) {
-    Con *focused_ws = con_get_workspace(focused);
-    Con *output;
-    uint32_t idx = 0;
-    /* We count to get the index of this workspace because named workspaces
-     * don’t have the ->num property */
-    TAILQ_FOREACH(output, &(croot->nodes_head), nodes) {
-        Con *ws;
-        TAILQ_FOREACH(ws, &(output_get_content(output)->nodes_head), nodes) {
-            if (STARTS_WITH(ws->name, "__"))
-                continue;
-
-            if (ws == focused_ws) {
-                xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root,
-                                    A__NET_CURRENT_DESKTOP, XCB_ATOM_CARDINAL, 32, 1, &idx);
-                return;
-            }
-            ++idx;
-        }
+    const uint32_t idx = ewmh_get_workspace_index(focused);
+    if (idx != NET_WM_DESKTOP_NONE) {
+        xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_CURRENT_DESKTOP, XCB_ATOM_CARDINAL, 32, 1, &idx);
     }
 }
 
@@ -138,6 +123,71 @@ void ewmh_update_desktop_viewport(void) {
                         A__NET_DESKTOP_VIEWPORT, XCB_ATOM_CARDINAL, 32, current_position, &viewports);
 }
 
+static void ewmh_update_wm_desktop_recursively(Con *con, const uint32_t desktop) {
+    /* Recursively call this to descend through the entire subtree. */
+    Con *child;
+    TAILQ_FOREACH(child, &(con->nodes_head), nodes) {
+        ewmh_update_wm_desktop_recursively(child, desktop);
+    }
+    /* If con is a workspace, we also need to go through the floating windows on it. */
+    if (con->type == CT_WORKSPACE) {
+        TAILQ_FOREACH(child, &(con->floating_head), floating_windows) {
+            ewmh_update_wm_desktop_recursively(child, desktop);
+        }
+    }
+
+    if (!con_has_managed_window(con))
+        return;
+
+    const xcb_window_t window = con->window->id;
+
+    uint32_t wm_desktop = desktop;
+    /* Sticky windows are only actually sticky when they are floating or inside
+     * a floating container. This is technically still slightly wrong, since
+     * sticky windows will only be on all workspaces on this output, but we
+     * ignore multi-monitor situations for this since the spec isn't too
+     * precise on this anyway. */
+    if (con_is_sticky(con) && con_is_floating(con)) {
+        wm_desktop = NET_WM_DESKTOP_ALL;
+    }
+
+    /* If this is the cached value, we don't need to do anything. */
+    if (con->window->wm_desktop == wm_desktop)
+        return;
+    con->window->wm_desktop = wm_desktop;
+
+    if (wm_desktop != NET_WM_DESKTOP_NONE) {
+        DLOG("Setting _NET_WM_DESKTOP = %d for window 0x%08x.\n", wm_desktop, window);
+        xcb_change_property(conn, XCB_PROP_MODE_REPLACE, window, A__NET_WM_DESKTOP, XCB_ATOM_CARDINAL, 32, 1, &wm_desktop);
+    } else {
+        /* If we can't determine the workspace index, delete the property. We'd
+         * rather not set it than lie. */
+        ELOG("Failed to determine the proper EWMH desktop index for window 0x%08x, deleting _NET_WM_DESKTOP.\n", window);
+        xcb_delete_property(conn, window, A__NET_WM_DESKTOP);
+    }
+}
+
+/*
+ * Updates _NET_WM_DESKTOP for all windows.
+ * A request will only be made if the cached value differs from the calculated value.
+ *
+ */
+void ewmh_update_wm_desktop(void) {
+    uint32_t desktop = 0;
+
+    Con *output;
+    TAILQ_FOREACH(output, &(croot->nodes_head), nodes) {
+        Con *workspace;
+        TAILQ_FOREACH(workspace, &(output_get_content(output)->nodes_head), nodes) {
+            if (con_is_internal(workspace))
+                continue;
+
+            ewmh_update_wm_desktop_recursively(workspace, desktop);
+            ++desktop;
+        }
+    }
+}
+
 /*
  * Updates _NET_ACTIVE_WINDOW with the currently focused window.
  *
@@ -270,3 +320,61 @@ void ewmh_setup_hints(void) {
     xcb_map_window(conn, ewmh_window);
     xcb_configure_window(conn, ewmh_window, XCB_CONFIG_WINDOW_STACK_MODE, (uint32_t[]){XCB_STACK_MODE_BELOW});
 }
+
+/*
+ * Returns the workspace container as enumerated by the EWMH desktop model.
+ * Returns NULL if no workspace could be found for the index.
+ *
+ * This is the reverse of ewmh_get_workspace_index.
+ *
+ */
+Con *ewmh_get_workspace_by_index(uint32_t idx) {
+    if (idx == NET_WM_DESKTOP_NONE)
+        return NULL;
+
+    uint32_t current_index = 0;
+
+    Con *output;
+    TAILQ_FOREACH(output, &(croot->nodes_head), nodes) {
+        Con *workspace;
+        TAILQ_FOREACH(workspace, &(output_get_content(output)->nodes_head), nodes) {
+            if (con_is_internal(workspace))
+                continue;
+
+            if (current_index == idx)
+                return workspace;
+
+            ++current_index;
+        }
+    }
+
+    return NULL;
+}
+
+/*
+ * Returns the EWMH desktop index for the workspace the given container is on.
+ * Returns NET_WM_DESKTOP_NONE if the desktop index cannot be determined.
+ *
+ * This is the reverse of ewmh_get_workspace_by_index.
+ *
+ */
+uint32_t ewmh_get_workspace_index(Con *con) {
+    uint32_t index = 0;
+
+    Con *workspace = con_get_workspace(con);
+    Con *output;
+    TAILQ_FOREACH(output, &(croot->nodes_head), nodes) {
+        Con *current;
+        TAILQ_FOREACH(current, &(output_get_content(output)->nodes_head), nodes) {
+            if (con_is_internal(current))
+                continue;
+
+            if (current == workspace)
+                return index;
+
+            ++index;
+        }
+    }
+
+    return NET_WM_DESKTOP_NONE;
+}