feature: implement ewmh desktop viewport property

Set and update the _NET_DESKTOP_VIEWPORT property

http://standards.freedesktop.org/wm-spec/wm-spec-latest.html#idm140146176862048

> _NET_DESKTOP_VIEWPORT x, y, CARDINAL[][2]/32
> Array of pairs of cardinals that define the top left corner of each
> desktop's viewport.

include/atoms.xmacro

@@ -17,6 +17,7 @@ xmacro(_NET_CLIENT_LIST)
 xmacro(_NET_CLIENT_LIST_STACKING)
 xmacro(_NET_CURRENT_DESKTOP)
 xmacro(_NET_NUMBER_OF_DESKTOPS)
+xmacro(_NET_DESKTOP_VIEWPORT)
 xmacro(_NET_ACTIVE_WINDOW)
 xmacro(_NET_STARTUP_ID)
 xmacro(_NET_WORKAREA)

include/ewmh.h

@@ -24,6 +24,12 @@ void ewmh_update_current_desktop(void);
  */
 void ewmh_update_number_of_desktops(void);
 
+/**
+ * Updates _NET_DESKTOP_VIEWPORT, which is an array of pairs of cardinals that
+ * define the top left corner of each desktop's viewport.
+ */
+void ewmh_update_desktop_viewport(void);
+
 /**
  * Updates _NET_ACTIVE_WINDOW with the currently focused window.
  *

src/ewmh.c

@@ -61,6 +61,43 @@ void ewmh_update_number_of_desktops(void) {
                         A__NET_NUMBER_OF_DESKTOPS, XCB_ATOM_CARDINAL, 32, 1, &idx);
 }
 
+/*
+ * Updates _NET_DESKTOP_VIEWPORT, which is an array of pairs of cardinals that
+ * define the top left corner of each desktop's viewport.
+ */
+void ewmh_update_desktop_viewport(void) {
+    Con *output;
+    int num_desktops = 0;
+    /* count number of desktops */
+    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;
+
+            num_desktops++;
+        }
+    }
+
+    uint32_t viewports[num_desktops * 2];
+
+    int current_position = 0;
+    /* fill the viewport buffer */
+    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;
+
+            viewports[current_position++] = output->rect.x;
+            viewports[current_position++] = output->rect.y;
+        }
+    }
+
+    xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root,
+                        A__NET_DESKTOP_VIEWPORT, XCB_ATOM_CARDINAL, 32, current_position, &viewports);
+}
+
 /*
  * Updates _NET_ACTIVE_WINDOW with the currently focused window.
  *
@@ -159,5 +196,5 @@ void ewmh_setup_hints(void) {
     /* I’m not entirely sure if we need to keep _NET_WM_NAME on root. */
     xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_WM_NAME, A_UTF8_STRING, 8, strlen("i3"), "i3");
 
-    xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_SUPPORTED, XCB_ATOM_ATOM, 32, 19, supported_atoms);
+    xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_SUPPORTED, XCB_ATOM_ATOM, 32, 20, supported_atoms);
 }

src/main.c

@@ -674,6 +674,7 @@ int main(int argc, char *argv[]) {
     /* Set the ewmh desktop properties. */
     ewmh_update_current_desktop();
     ewmh_update_number_of_desktops();
+    ewmh_update_desktop_viewport();
 
     struct ev_io *xcb_watcher = scalloc(sizeof(struct ev_io));
     xcb_check = scalloc(sizeof(struct ev_check));

src/workspace.c

@@ -93,6 +93,7 @@ Con *workspace_get(const char *num, bool *created) {
 
         ipc_send_event("workspace", I3_IPC_EVENT_WORKSPACE, "{\"change\":\"init\"}");
         ewmh_update_number_of_desktops();
+        ewmh_update_desktop_viewport();
         if (created != NULL)
             *created = true;
     } else if (created != NULL) {
@@ -418,6 +419,7 @@ static void _workspace_show(Con *workspace) {
             tree_close(old, DONT_KILL_WINDOW, false, false);
             ipc_send_event("workspace", I3_IPC_EVENT_WORKSPACE, "{\"change\":\"empty\"}");
             ewmh_update_number_of_desktops();
+            ewmh_update_desktop_viewport();
         }
     }