Use tree-sitter Tree.changed_ranges.

A first attempt at using changed_ranges to reduce the amount of
redrawing triggered by completion of a tree-sitter incremental parse.

Author: paul-ollis

src/textual/document/_syntax_aware_document.py

@@ -239,9 +239,12 @@ def reparse(self, timeout_us: int, lines: list[str], syntax_tree=None) -> bool:
                 # The only known cause is a timeout.
                 return False
             else:
-                self._syntax_tree = tree
                 if self._syntax_tree_update_callback is not None:
-                    self._syntax_tree_update_callback()
+                    changed_ranges = self._syntax_tree.changed_ranges(tree)
+                    self._syntax_tree = tree
+                    self._syntax_tree_update_callback(changed_ranges)
+                else:
+                    self._syntax_tree = tree
                 return True
         finally:
             self._parser.timeout_micros = saved_timeout

src/textual/widgets/_text_area.py

@@ -708,14 +708,62 @@ def check_consume_key(self, key: str, character: str | None = None) -> bool:
         # Otherwise we capture all printable keys
         return character is not None and character.isprintable()
 
-    def _handle_syntax_tree_update(self) -> None:
+    def _handle_syntax_tree_update(self, tree_ranges) -> None:
         """Reflect changes to the syntax tree."""
-        if self._highlight_query:
-            self._highlights.reset()
+        if not self._highlight_query:
+            return
+
+        self._highlights.reset()
+
+        _, first_line_index = self.scroll_offset
+        visible_line_range = range(
+            first_line_index, first_line_index + self.size.height
+        )
 
-            # TODO: This feels heavy handed.
-            _, scroll_offset_y = self.scroll_offset
-            self.refresh(Region(0, scroll_offset_y, self.size.width, self.size.height))
+        visible_region = self.window_region
+        regions = []
+        full_width = self.size.width
+
+        for tree_range in tree_ranges:
+            start_row = tree_range.start_point.row
+            end_row = tree_range.end_point.row
+            if not (start_row in visible_line_range or end_row in visible_line_range):
+                continue
+
+            start_column = tree_range.start_point.column
+            end_column = tree_range.end_point.column
+
+            if start_row == end_row:
+                width = end_column = start_column
+                tree_region = Region(start_column, start_row, width, 1)
+                region = tree_region.intersection(visible_region)
+                if region.area > 0:
+                    regions.append(region)
+
+            else:
+                # Add region for the first changed line.
+                width = full_width - start_column
+                tree_region = Region(start_column, start_row, width, 1)
+                region = tree_region.intersection(visible_region)
+                if region.area > 0:
+                    regions.append(region)
+
+                # Add region for the last changed line.
+                tree_region = Region(0, end_row, end_column, 1)
+                region = tree_region.intersection(visible_region)
+                if region.area > 0:
+                    regions.append(region)
+
+                # Add region for the other lines.
+                height = end_row - start_row - 1
+                if height > 0:
+                    tree_region = Region(0, start_row + 1, width, height)
+                    region = tree_region.intersection(visible_region)
+                    if region.area > 0:
+                        regions.append(region)
+
+            if regions:
+                self.refresh(*regions)
 
     def _handle_change_affecting_highlighting(
         self,