Fix panic from two windows editing the same document (#4570)

* Clamp highlighting range to be within document

This fixes a panic possible when two vsplits of the same document
exist and enough lines are deleted from the document so that one of
the windows focuses past the end of the document.

* 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`).

* Update helix-term/src/ui/editor.rs

Co-authored-by: Blaž Hrastnik <blaz@mxxn.io>

Co-authored-by: Blaž Hrastnik <blaz@mxxn.io>

helix-term/src/ui/editor.rs

@@ -227,16 +227,16 @@ impl EditorView {
         _theme: &Theme,
     ) -> Box<dyn Iterator<Item = HighlightEvent> + 'doc> {
         let text = doc.text().slice(..);
-        let last_line = std::cmp::min(
-            // Saturating subs to make it inclusive zero indexing.
-            (offset.row + height as usize).saturating_sub(1),
-            doc.text().len_lines().saturating_sub(1),
-        );
 
         let range = {
-            // calculate viewport byte ranges
-            let start = text.line_to_byte(offset.row);
-            let end = text.line_to_byte(last_line + 1);
+            // Calculate viewport byte ranges:
+            // Saturating subs to make it inclusive zero indexing.
+            let last_line = doc.text().len_lines().saturating_sub(1);
+            let last_visible_line = (offset.row + height as usize)
+                .saturating_sub(1)
+                .min(last_line);
+            let start = text.line_to_byte(offset.row.min(last_line));
+            let end = text.line_to_byte(last_visible_line + 1);
 
             start..end
         };

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);
         }
     }