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.

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.min(last_line) + 1);
 
             start..end
         };