fix CloseCurrentPane leaving a stranded pane in a tab

Repro for this issue was:

```console
$ WEZTERM_LOG=mux=trace,info ./target/debug/wezterm -n --config 'exit_behavior="CloseOnCleanExit"'
```

Then:

* Split left/right
* CloseCurrentPane

refs: #4030

docs/changelog.md

@@ -92,6 +92,7 @@ As features stabilize some brief notes about them will accumulate here.
 * Text cursor filled the scaled-by `line_height` and `cell_width` dimensions rather
   than the native font dimensions and looked weird when either config option was
   not set to `1.0`. #2882
+* Using `CloseCurrentPane` could sometimes leave a stranded pane in a tab. #4030
 
 #### Updated
 * Bundled harfbuzz to 8.1.1

mux/src/tab.rs

@@ -1554,8 +1554,26 @@ impl TabInner {
     }
 
     fn prune_dead_panes(&mut self) -> bool {
+        let mux = Mux::get();
         !self
-            .remove_pane_if(|_, pane| pane.is_dead(), true)
+            .remove_pane_if(
+                |_, pane| {
+                    // If the pane is no longer known to the mux, then its liveness
+                    // state isn't guaranteed to be monitored or updated, so let's
+                    // consider the pane effectively dead if it isn't in the mux.
+                    // <https://github.com/wez/wezterm/issues/4030>
+                    let in_mux = mux.get_pane(pane.pane_id()).is_some();
+                    let dead = pane.is_dead();
+                    log::trace!(
+                        "prune_dead_panes: pane_id={} dead={} in_mux={}",
+                        pane.pane_id(),
+                        dead,
+                        in_mux
+                    );
+                    dead || !in_mux
+                },
+                true,
+            )
             .is_empty()
     }