wezterm: add CloseActivePane key assignment

This removes the active pane from the current tab, causing the
tab to close if it was the last remaining pane in the tab.

```lua
  {key="W", mods="CTRL", action="CloseActivePane"},
```

refs: https://github.com/wez/wezterm/issues/157

src/frontend/gui/termwindow.rs

@@ -1855,6 +1855,7 @@ impl TermWindow {
                 }
             }
             CloseCurrentTab => self.close_current_tab(),
+            CloseActivePane => self.close_active_pane(),
             Nop | DisableDefaultAssignment => {}
             ReloadConfiguration => crate::config::reload(),
             MoveTab(n) => self.move_tab(*n)?,
@@ -2125,6 +2126,15 @@ impl TermWindow {
         self.scaling_changed(self.dimensions, 1.);
     }
 
+    fn close_active_pane(&mut self) {
+        let mux = Mux::get().unwrap();
+        let tab = match mux.get_active_tab_for_window(self.mux_window_id) {
+            Some(tab) => tab,
+            None => return,
+        };
+        tab.kill_active_pane();
+    }
+
     fn close_current_tab(&mut self) {
         let mux = Mux::get().unwrap();
         let tab = match mux.get_active_tab_for_window(self.mux_window_id) {

src/keyassignment.rs

@@ -123,6 +123,7 @@ pub enum KeyAssignment {
     AdjustPaneSize(PaneDirection, usize),
     ActivatePaneDirection(PaneDirection),
     TogglePaneZoomState,
+    CloseActivePane,
 }
 impl_lua_conversion!(KeyAssignment);
 

src/mux/tab.rs

@@ -711,6 +711,18 @@ impl Tab {
     }
 
     pub fn prune_dead_panes(&self) -> bool {
+        self.remove_pane_if(|_, pane| pane.is_dead())
+    }
+
+    pub fn kill_active_pane(&self) -> bool {
+        let active_idx = *self.active.borrow();
+        self.remove_pane_if(|idx, _| idx == active_idx)
+    }
+
+    fn remove_pane_if<F>(&self, f: F) -> bool
+    where
+        F: Fn(usize, &Rc<dyn Pane>) -> bool,
+    {
         let mut dead_panes = vec![];
 
         {
@@ -723,7 +735,7 @@ impl Tab {
 
             loop {
                 // Figure out the available size by looking at our immediate parent node.
-                // If we are the root, look at the provided new size
+                // If we are the root, look at the tab size
                 let pane_size = if let Some((branch, Some(parent))) = cursor.path_to_root().next() {
                     if branch == PathBranch::IsRight {
                         parent.second
@@ -736,7 +748,7 @@ impl Tab {
 
                 if cursor.is_leaf() {
                     let pane = Rc::clone(cursor.leaf_mut().unwrap());
-                    if pane.is_dead() {
+                    if f(pane_index, &pane) {
                         if pane_index == active_idx {
                             active_idx = pane_index.saturating_sub(1);
                         }
@@ -749,7 +761,11 @@ impl Tab {
                             }
                             Err(c) => {
                                 // We might be the root, for example
-                                root.replace(c.tree());
+                                if c.is_top() && c.is_leaf() {
+                                    root.replace(Tree::Empty);
+                                } else {
+                                    root.replace(c.tree());
+                                }
                                 break;
                             }
                         };