Only store one nav history entry when opening excerpts

Also: Introduce the ability to disable and enable the nav history directly. This allows us to explicitly push an entry when opening excerpts and then disable all pushes as we open individual buffers.

crates/workspace/src/pane.rs

@@ -123,6 +123,7 @@ enum NavigationMode {
     Normal,
     GoingBack,
     GoingForward,
+    Disabled,
 }
 
 impl Default for NavigationMode {
@@ -149,7 +150,7 @@ impl Pane {
         }
     }
 
-    pub(crate) fn nav_history(&self) -> &Rc<RefCell<NavHistory>> {
+    pub fn nav_history(&self) -> &Rc<RefCell<NavHistory>> {
         &self.nav_history
     }
 
@@ -649,14 +650,6 @@ impl<T: Toolbar> ToolbarHandle for ViewHandle<T> {
 }
 
 impl ItemNavHistory {
-    pub fn len(&self) -> usize {
-        self.history.borrow().len()
-    }
-
-    pub fn truncate(&self, len: usize) {
-        self.history.borrow_mut().truncate(len)
-    }
-
     pub fn new<T: ItemView>(history: Rc<RefCell<NavHistory>>, item_view: &ViewHandle<T>) -> Self {
         Self {
             history,
@@ -674,12 +667,12 @@ impl ItemNavHistory {
 }
 
 impl NavHistory {
-    pub fn len(&self) -> usize {
-        self.backward_stack.len()
+    pub fn disable(&mut self) {
+        self.mode = NavigationMode::Disabled;
     }
 
-    pub fn truncate(&mut self, len: usize) {
-        self.backward_stack.truncate(len);
+    pub fn enable(&mut self) {
+        self.mode = NavigationMode::Normal;
     }
 
     pub fn pop_backward(&mut self) -> Option<NavigationEntry> {
@@ -692,7 +685,7 @@ impl NavHistory {
 
     fn pop(&mut self, mode: NavigationMode) -> Option<NavigationEntry> {
         match mode {
-            NavigationMode::Normal => None,
+            NavigationMode::Normal | NavigationMode::Disabled => None,
             NavigationMode::GoingBack => self.pop_backward(),
             NavigationMode::GoingForward => self.pop_forward(),
         }
@@ -708,6 +701,7 @@ impl NavHistory {
         item_view: Rc<dyn WeakItemViewHandle>,
     ) {
         match self.mode {
+            NavigationMode::Disabled => {}
             NavigationMode::Normal => {
                 if self.backward_stack.len() >= MAX_NAVIGATION_HISTORY_LEN {
                     self.backward_stack.pop_front();