Ensure cursor is in view on window change

If two windows are editing the same document, one may delete enough of
the document so that the other window is pointing at a blank page (past
the document end). In this change we ensure that the cursor is within
view whenever we switch to a new window (for example with `<C-w>w`).

helix-view/src/editor.rs

@@ -1223,9 +1223,11 @@ impl Editor {
     pub fn focus(&mut self, view_id: ViewId) {
         let prev_id = std::mem::replace(&mut self.tree.focus, view_id);
 
-        // if leaving the view: mode should reset
+        // if leaving the view: mode should reset and the cursor should be
+        // within view
         if prev_id != view_id {
             self.mode = Mode::Normal;
+            self.ensure_cursor_in_view(view_id);
         }
     }
 
@@ -1234,9 +1236,11 @@ impl Editor {
         self.tree.focus_next();
         let id = self.tree.focus;
 
-        // if leaving the view: mode should reset
+        // if leaving the view: mode should reset and the cursor should be
+        // within view
         if prev_id != id {
             self.mode = Mode::Normal;
+            self.ensure_cursor_in_view(id);
         }
     }