Fix pane focus rapidly switching back and forth

Switching panes multiple times too quickly sometimes causes the focus to
get stuck rapidly switching back and forth between two panes by itself.
This happens both locally and on an SSH domain. The root cause is
similar in both cases.

In the local case, GuiFrontend handles a MuxNotification::PaneFocused
event by calling Mux::focus_pane_and_containing_tab, which calls through
Tab::set_active_pane -> TabInner::set_active_pane ->
TabInner::advise_focus_change. TabInner::advise_focus_change then calls
Mux::notify with a new MuxNotification::PaneFocused event, which is a
redundant notification focusing the same pane again.

This is normally harmless other than a small amount of wasted work;
TabInner::set_active_pane notices that the focused pane didn't change
and doesn't generate yet another event.

However, if another real focus change is queued between the original
focus change and the redundant one, we get into trouble. For example, if
the user switches to pane 0 and then immediately to pane 1, the event
queue contains [PaneFocused(0), PaneFocused(1)]. When the first event is
handled, pane 0 is focused, and a redundant notification is generated,
so the event queue contains [PaneFocused(1), PaneFocused(0)]. Then after
the next event is handled, pane 1 is focused, and we add another
redundant notification, so the event queue contains [PaneFocused(0),
PaneFocused(1)]. Repeat ad nauseam.

Fix this by not generating the notification from
TabInner::advise_focus_change when it would be redundant, which we
indicate with a new boolean parameter.

The mux server case is very similar, except that Pdu::SetFocusedPane in
SessionHandler is the vector for the bug.

This bug appears to have been introduced by commit
f71bce1. I verified that `wezterm cli
activate-pane-direction` still works over SSH after this fix.

closes: #4390
closes: #4693

lua-api-crates/mux/src/pane.rs

@@ -1,6 +1,7 @@
 use super::*;
 use luahelper::{dynamic_to_lua_value, from_lua, to_lua};
 use mlua::Value;
+use mux::tab::NotifyMux;
 use std::cmp::Ordering;
 use std::sync::Arc;
 use termwiz::cell::SemanticType;
@@ -400,7 +401,7 @@ impl UserData for MuxPane {
             let tab = mux
                 .get_tab(tab_id)
                 .ok_or_else(|| mlua::Error::external(format!("tab {tab_id} not found")))?;
-            tab.set_active_pane(&pane);
+            tab.set_active_pane(&pane, NotifyMux::Yes);
             Ok(())
         });
 

mux/src/lib.rs

@@ -1,6 +1,6 @@
 use crate::client::{ClientId, ClientInfo};
 use crate::pane::{Pane, PaneId};
-use crate::tab::{SplitRequest, Tab, TabId};
+use crate::tab::{NotifyMux, SplitRequest, Tab, TabId};
 use crate::window::{Window, WindowId};
 use anyhow::{anyhow, Context, Error};
 use config::keyassignment::SpawnTabDomain;
@@ -542,7 +542,7 @@ impl Mux {
             .get_tab(tab_id)
             .ok_or_else(|| anyhow::anyhow!("tab {tab_id} not found"))?;
 
-        tab.set_active_pane(&pane);
+        tab.set_active_pane(&pane, NotifyMux::No);
 
         Ok(())
     }

mux/src/tab.rs

@@ -20,6 +20,12 @@ pub type Cursor = bintree::Cursor<Arc<dyn Pane>, SplitDirectionAndSize>;
 static TAB_ID: ::std::sync::atomic::AtomicUsize = ::std::sync::atomic::AtomicUsize::new(0);
 pub type TabId = usize;
 
+#[derive(Debug, Clone, Copy, Eq, PartialEq)]
+pub enum NotifyMux {
+    Yes,
+    No,
+}
+
 #[derive(Default)]
 struct Recency {
     count: usize,
@@ -698,8 +704,8 @@ impl Tab {
         self.inner.lock().get_active_idx()
     }
 
-    pub fn set_active_pane(&self, pane: &Arc<dyn Pane>) {
-        self.inner.lock().set_active_pane(pane)
+    pub fn set_active_pane(&self, pane: &Arc<dyn Pane>, notify: NotifyMux) {
+        self.inner.lock().set_active_pane(pane, notify)
     }
 
     pub fn set_active_idx(&self, pane_index: usize) {
@@ -1747,7 +1753,7 @@ impl TabInner {
         self.active
     }
 
-    fn set_active_pane(&mut self, pane: &Arc<dyn Pane>) {
+    fn set_active_pane(&mut self, pane: &Arc<dyn Pane>, notify: NotifyMux) {
         if let Some(item) = self
             .iter_panes_ignoring_zoom()
             .iter()
@@ -1756,22 +1762,26 @@ impl TabInner {
             let prior = self.get_active_pane();
             self.active = item.index;
             self.recency.tag(item.index);
-            self.advise_focus_change(prior);
+            self.advise_focus_change(prior, notify);
         }
     }
 
-    fn advise_focus_change(&mut self, prior: Option<Arc<dyn Pane>>) {
+    fn advise_focus_change(&mut self, prior: Option<Arc<dyn Pane>>, notify: NotifyMux) {
         let mux = Mux::get();
         let current = self.get_active_pane();
         match (prior, current) {
             (Some(prior), Some(current)) if prior.pane_id() != current.pane_id() => {
                 prior.focus_changed(false);
                 current.focus_changed(true);
-                mux.notify(MuxNotification::PaneFocused(current.pane_id()));
+                if notify == NotifyMux::Yes {
+                    mux.notify(MuxNotification::PaneFocused(current.pane_id()));
+                }
             }
             (None, Some(current)) => {
                 current.focus_changed(true);
-                mux.notify(MuxNotification::PaneFocused(current.pane_id()));
+                if notify == NotifyMux::Yes {
+                    mux.notify(MuxNotification::PaneFocused(current.pane_id()));
+                }
             }
             (Some(prior), None) => {
                 prior.focus_changed(false);
@@ -1786,7 +1796,7 @@ impl TabInner {
         let prior = self.get_active_pane();
         self.active = pane_index;
         self.recency.tag(pane_index);
-        self.advise_focus_change(prior);
+        self.advise_focus_change(prior, NotifyMux::Yes);
     }
 
     fn assign_pane(&mut self, pane: &Arc<dyn Pane>) {
@@ -1849,7 +1859,7 @@ impl TabInner {
         if keep_focus {
             self.set_active_idx(pane_index);
         } else {
-            self.advise_focus_change(Some(pane));
+            self.advise_focus_change(Some(pane), NotifyMux::Yes);
         }
         None
     }

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

@@ -6,7 +6,7 @@ use mux::client::ClientId;
 use mux::domain::SplitSource;
 use mux::pane::{Pane, PaneId};
 use mux::renderable::{RenderableDimensions, StableCursorPosition};
-use mux::tab::TabId;
+use mux::tab::{NotifyMux, TabId};
 use mux::{Mux, MuxNotification};
 use promise::spawn::spawn_into_main_thread;
 use std::collections::HashMap;
@@ -332,10 +332,9 @@ impl SessionHandler {
                             let tab = mux
                                 .get_tab(tab_id)
                                 .ok_or_else(|| anyhow::anyhow!("tab {tab_id} not found"))?;
-                            tab.set_active_pane(&pane);
+                            tab.set_active_pane(&pane, NotifyMux::No);
 
                             mux.record_focus_for_current_identity(pane_id);
-                            mux.notify(mux::MuxNotification::PaneFocused(pane_id));
 
                             Ok(Pdu::UnitResponse(UnitResponse {}))
                         },
@@ -538,14 +537,14 @@ impl SessionHandler {
                                     if is_zoomed != zoomed {
                                         tab.set_zoomed(false);
                                         if zoomed {
-                                            tab.set_active_pane(&pane);
+                                            tab.set_active_pane(&pane, NotifyMux::Yes);
                                             tab.set_zoomed(zoomed);
                                         }
                                     }
                                 }
                                 None => {
                                     if zoomed {
-                                        tab.set_active_pane(&pane);
+                                        tab.set_active_pane(&pane, NotifyMux::Yes);
                                         tab.set_zoomed(zoomed);
                                     }
                                 }