add window:is_focused() method

refs: https://github.com/wez/wezterm/discussions/2537

docs/changelog.md

@@ -34,6 +34,9 @@ As features stabilize some brief notes about them will accumulate here.
 * [wezterm cli activate-pane-direction](cli/cli/activate-pane-direction.md)
   command. Thanks to [@abusch](https://github.com/abusch)!
   [#2526](https://github.com/wez/wezterm/pull/2526)
+* [window:is_focused()](config/lua/window/is_focused.md) method for testing
+  whether a GUI window has focus.
+  [#2537](https://github.com/wez/wezterm/discussions/2537)
 
 #### Fixed
 * Wayland: key repeat gets stuck after pressing two keys in quick succession.

docs/config/lua/window/is_focused.md

@@ -0,0 +1,26 @@
+# wezterm:is_focused()
+
+*Since: nightly builds only*
+
+Returns `true` if the window has focus.
+
+The `update-status` event is fired when the focus state changes.
+
+This example changes the color scheme based on the focus state:
+
+```lua
+local wezterm = require 'wezterm'
+
+wezterm.on('update-status', function(window, pane)
+  local overrides = window:get_config_overrides() or {}
+  if window:is_focused() then
+    overrides.color_scheme = 'nordfox'
+  else
+    overrides.color_scheme = 'nightfox'
+  end
+  window:set_config_overrides(overrides)
+end)
+
+return {}
+```
+

wezterm-gui/src/scripting/guiwin.rs

@@ -184,6 +184,20 @@ impl UserData for GuiWin {
                 .notify(TermWindowNotif::SetConfigOverrides(value));
             Ok(())
         });
+        methods.add_async_method("is_focused", |_, this, _: ()| async move {
+            let (tx, rx) = smol::channel::bounded(1);
+            this.window
+                .notify(TermWindowNotif::Apply(Box::new(move |term_window| {
+                    tx.try_send(term_window.focused.is_some()).ok();
+                })));
+            let result = rx
+                .recv()
+                .await
+                .map_err(|e| anyhow::anyhow!("{:#}", e))
+                .map_err(luaerr)?;
+
+            Ok(result)
+        });
         methods.add_async_method("leader_is_active", |_, this, _: ()| async move {
             let (tx, rx) = smol::channel::bounded(1);
             this.window

wezterm-gui/src/termwindow/mod.rs

@@ -351,7 +351,7 @@ pub struct TermWindow {
     pub config_overrides: wezterm_dynamic::Value,
     os_parameters: Option<parameters::Parameters>,
     /// When we most recently received keyboard focus
-    focused: Option<Instant>,
+    pub focused: Option<Instant>,
     fonts: Rc<FontConfiguration>,
     /// Window dimensions and dpi
     pub dimensions: Dimensions,
@@ -525,6 +525,8 @@ impl TermWindow {
         if let Some(pane) = self.get_active_pane_or_overlay() {
             pane.focus_changed(focused);
         }
+
+        self.update_title();
     }
 
     fn created(