diff --git a/apps/main.c b/apps/main.c
index db3a33c..f746704 100644
--- a/apps/main.c
+++ b/apps/main.c
@@ -132,6 +132,7 @@ int main(void)
     apps_image_start(tx->screen, "Viewer", 20, 20);
 #endif
 
+    twin_screen_set_active(tx->screen, tx->screen->top);
     twin_dispatch(tx);
 
     return 0;
diff --git a/src/screen.c b/src/screen.c
index c3dfb2d..76bcfe2 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -151,25 +151,6 @@ static void twin_screen_span_pixmap(twin_screen_t maybe_unused *screen,
         op32(dst, src, p_right - p_left);
 }
 
-static twin_pixmap_t *twin_active_pixmap(twin_screen_t *screen,
-                                         twin_pixmap_t **active_pix)
-{
-    twin_pixmap_t *p = NULL, *prev_active_pix = NULL;
-    /*
-     * Identify the previously active pixel map and the currently active pixel
-     * map, which is on the topmost layer.
-     */
-    for (p = screen->bottom; p; p = p->up) {
-        if (p->window->active == true) {
-            prev_active_pix = p;
-            prev_active_pix->window->active = false;
-        }
-        (*active_pix) = p;
-    }
-    (*active_pix)->window->active = true;
-    return prev_active_pix;
-}
-
 void twin_screen_update(twin_screen_t *screen)
 {
     twin_coord_t left = screen->damage.left;
@@ -189,7 +170,7 @@ void twin_screen_update(twin_screen_t *screen)
 
     if (!screen->disable && left < right && top < bottom) {
         twin_argb32_t *span;
-        twin_pixmap_t *p, *active_pix = NULL, *prev_active_pix = NULL;
+        twin_pixmap_t *p;
         twin_coord_t y;
         twin_coord_t width = right - left;
 
@@ -202,24 +183,6 @@ void twin_screen_update(twin_screen_t *screen)
 
         if (screen->put_begin)
             (*screen->put_begin)(left, top, right, bottom, screen->closure);
-
-        prev_active_pix = twin_active_pixmap(screen, &active_pix);
-
-        /*
-         * Mark the previously active pixel map as damaged to update its
-         * changes.
-         */
-        if (prev_active_pix && active_pix != prev_active_pix) {
-            twin_pixmap_damage(prev_active_pix, 0, 0, prev_active_pix->width,
-                               prev_active_pix->height);
-            twin_window_draw(prev_active_pix->window);
-        }
-
-        /* Mark the active pixel map as damaged to update its changes. */
-        twin_pixmap_damage(active_pix, 0, 0, active_pix->width,
-                           active_pix->height);
-        twin_window_draw(active_pix->window);
-
         for (y = top; y < bottom; y++) {
             if (screen->background) {
                 twin_pointer_t dst;
diff --git a/src/window.c b/src/window.c
index 59696fd..3e2547a 100644
--- a/src/window.c
+++ b/src/window.c
@@ -387,6 +387,20 @@ bool twin_window_dispatch(twin_window_t *window, twin_event_t *event)
 
     switch (ev.kind) {
     case TwinEventButtonDown:
+    case TwinEventActivate:
+        /* Set window active. */
+        /*
+         * When the box is trigger by TwinEventButtonDown, its window's title
+         * bar needs to change color and be put onto the toppest layer.
+         */
+        if (!window->active) {
+            window->active = true;
+            twin_window_frame(window);
+            if (window != window->screen->top->window) {
+                window->screen->top->window->active = false;
+                twin_window_frame(window->screen->top->window);
+            }
+        }
         if (window->client.left <= ev.u.pointer.x &&
             ev.u.pointer.x < window->client.right &&
             window->client.top <= ev.u.pointer.y &&