mux: Fix mouse wheel cursor movement on remote alt screens

Track alternate screen state for panes connected to a mux domain. This
enables mouse wheel events to translate correctly to cursor movement
for alt screen applications.

closes: https://github.com/wezterm/wezterm/issues/4922

codec/src/lib.rs

@@ -442,7 +442,7 @@ macro_rules! pdu {
 /// The overall version of the codec.
 /// This must be bumped when backwards incompatible changes
 /// are made to the types and protocol.
-pub const CODEC_VERSION: usize = 45;
+pub const CODEC_VERSION: usize = 46;
 
 // Defines the Pdu enum.
 // Each struct has an explicit identifying number.
@@ -915,6 +915,7 @@ pub struct LivenessResponse {
 pub struct GetPaneRenderChangesResponse {
     pub pane_id: PaneId,
     pub mouse_grabbed: bool,
+    pub alt_screen_active: bool,
     pub cursor_position: StableCursorPosition,
     pub dimensions: RenderableDimensions,
     pub dirty_lines: Vec<Range<StableRowIndex>>,

docs/changelog.md

@@ -160,6 +160,10 @@ As features stabilize some brief notes about them will accumulate here.
   `CTRL-u` to kill back to the start of the line. Thanks to @bew! #8013
 
 #### Fixed
+* Multiplexer: the mouse wheel was not mapped to cursor up/down keys in the
+  alternate screen for panes in a `wezterm connect` domain, because the client
+  always reported the alternate screen as inactive. The server now advises the
+  client of the alternate screen state. #4922
 * macOS: Fix window border when opacity<1 and shadow enabled.
   Thanks to @Adams-Galaxy! #8038 #5158
 * perf: Terminal images were hashed three times each on the transmit path; the sha256

wezterm-client/src/pane/clientpane.rs

@@ -44,6 +44,7 @@ pub struct ClientPane {
     mouse: Arc<Mutex<MouseState>>,
     clipboard: Mutex<Option<Arc<dyn Clipboard>>>,
     mouse_grabbed: Mutex<bool>,
+    alt_screen_active: Mutex<bool>,
     ignore_next_kill: Mutex<bool>,
     user_vars: Mutex<HashMap<String, String>>,
     config: Mutex<Option<Arc<dyn TerminalConfiguration>>>,
@@ -126,6 +127,7 @@ impl ClientPane {
             palette: Mutex::new(palette),
             clipboard: Mutex::new(None),
             mouse_grabbed: Mutex::new(false),
+            alt_screen_active: Mutex::new(false),
             ignore_next_kill: Mutex::new(false),
             unseen_output: Mutex::new(false),
             user_vars: Mutex::new(HashMap::new()),
@@ -138,6 +140,7 @@ impl ClientPane {
         match pdu {
             Pdu::GetPaneRenderChangesResponse(mut delta) => {
                 *self.mouse_grabbed.lock() = delta.mouse_grabbed;
+                *self.alt_screen_active.lock() = delta.alt_screen_active;
 
                 let bonus_lines = std::mem::take(&mut delta.bonus_lines);
                 let client = { Arc::clone(&self.renderable.lock().inner.borrow().client) };
@@ -546,8 +549,7 @@ impl Pane for ClientPane {
     }
 
     fn is_alt_screen_active(&self) -> bool {
-        // FIXME: retrieve this from the remote
-        false
+        *self.alt_screen_active.lock()
     }
 
     fn get_current_working_dir(&self, _policy: CachePolicy) -> Option<Url> {

wezterm-mux-server-impl/src/sessionhandler.rs

@@ -42,6 +42,7 @@ pub(crate) struct PerPane {
     working_dir: Option<Url>,
     dimensions: RenderableDimensions,
     mouse_grabbed: bool,
+    alt_screen_active: bool,
     sent_initial_palette: bool,
     seqno: SequenceNo,
     config_generation: usize,
@@ -60,6 +61,11 @@ impl PerPane {
             changed = true;
         }
 
+        let alt_screen_active = pane.is_alt_screen_active();
+        if alt_screen_active != self.alt_screen_active {
+            changed = true;
+        }
+
         let dims = pane.get_dimensions();
         if dims != self.dimensions {
             changed = true;
@@ -126,11 +132,13 @@ impl PerPane {
         self.working_dir = working_dir.clone();
         self.dimensions = dims;
         self.mouse_grabbed = mouse_grabbed;
+        self.alt_screen_active = alt_screen_active;
 
         let bonus_lines = bonus_lines.into();
         Some(GetPaneRenderChangesResponse {
             pane_id: pane.pane_id(),
             mouse_grabbed,
+            alt_screen_active,
             dirty_lines: all_dirty_lines.iter().cloned().collect(),
             dimensions: dims,
             cursor_position,