diff --git a/src/textual/widgets/_text_area.py b/src/textual/widgets/_text_area.py
index 7f01093598..be38de37ee 100644
--- a/src/textual/widgets/_text_area.py
+++ b/src/textual/widgets/_text_area.py
@@ -81,48 +81,60 @@ class HighlightMap:
 
     BLOCK_SIZE = 50
 
-    def __init__(self, text_area_widget: widgets.TextArea):
-        self.text_area_widget: widgets.TextArea = text_area_widget
-        self.uncovered_lines: dict[int, range] = {}
+    def __init__(self, text_area: TextArea):
+        self.text_area: TextArea = text_area
+        """The text area associated with this highlight map."""
 
-        # A mapping from line index to a list of Highlight instances.
-        self._highlights: LineToHighlightsMap = defaultdict(list)
-        self.reset()
+        self._highlighted_blocks: set[int] = set()
+        """The set of blocks that have been highlighted, identified by the start line index of the block.
+        (0 represents the first block, 50 the second, 100 the third, etc. - assuming a block size of 50)
+        """
+
+        self._highlights: dict[int, list[Highlight]] = defaultdict(list)
+        """A mapping from line index to a list of Highlight instances."""
 
     def reset(self) -> None:
         """Reset so that future lookups rebuild the highlight map."""
         self._highlights.clear()
-        line_count = self.document.line_count
-        uncovered_lines = self.uncovered_lines
-        uncovered_lines.clear()
-        i = end_range = 0
-        for i in range(0, line_count, self.BLOCK_SIZE):
-            end_range = min(i + self.BLOCK_SIZE, line_count)
-            line_range = range(i, end_range)
-            uncovered_lines.update({j: line_range for j in line_range})
-        if end_range < line_count:
-            line_range = range(i, line_count)
-            uncovered_lines.update({j: line_range for j in line_range})
+        self._highlighted_blocks.clear()
 
     @property
     def document(self) -> DocumentBase:
         """The text document being highlighted."""
-        return self.text_area_widget.document
+        return self.text_area.document
+
+    def __getitem__(self, index: int) -> list[Highlight]:
+        start, end = self._get_block_boundaries(index, self.BLOCK_SIZE)
+        if start not in self._highlighted_blocks:
+            self._highlighted_blocks.add(start)
+            self._build_part_of_highlight_map(range(start, end))
+        return self._highlights[index]
 
-    def __getitem__(self, idx: int) -> list[text_area.Highlight]:
-        if idx in self.uncovered_lines:
-            self._build_part_of_highlight_map(self.uncovered_lines[idx])
-        return self._highlights[idx]
+    def _get_block_boundaries(self, index: int, block_size: int) -> tuple[int, int]:
+        """Get the start and end of the block for the given index.
+
+        The start is inclusive and the end is exclusive.
+
+        Args:
+            index: The line index to get we want to know the block range for..
+            block_size: The size of the bucket.
+
+        Returns:
+            A tuple containing the start and end of the block.
+        """
+        block_index = index // block_size
+        start = block_index * block_size
+        end = (block_index + 1) * block_size
+        return (start, end)
 
     def _build_part_of_highlight_map(self, line_range: range) -> None:
         """Build part of the highlight map."""
         highlights = self._highlights
-        for line_index in line_range:
-            self.uncovered_lines.pop(line_index)
+
         start_point = (line_range[0], 0)
         end_point = (line_range[-1] + 1, 0)
         captures = self.document.query_syntax_tree(
-            self.text_area_widget._highlight_query,
+            self.text_area._highlight_query,
             start_point=start_point,
             end_point=end_point,
         )
@@ -140,8 +152,9 @@ def _build_part_of_highlight_map(self, line_range: range) -> None:
                     )
 
                     # Add the middle lines - entire row of this node is highlighted
+                    middle_highlight = (0, None, highlight_name)
                     for node_row in range(node_start_row + 1, node_end_row):
-                        highlights[node_row].append((0, None, highlight_name))
+                        highlights[node_row].append(middle_highlight)
 
                     # Add the last line of the node range
                     highlights[node_end_row].append(
@@ -157,16 +170,16 @@ def _build_part_of_highlight_map(self, line_range: range) -> None:
         # to be sorted in ascending order of ``a``. When two highlights have the same
         # value of ``a`` then the one with the larger a--b range comes first, with ``None``
         # being considered larger than any number.
-        def sort_key(hl) -> tuple[int, int, int]:
-            a, b, _ = hl
-            max_range_ind = 1
+        def sort_key(highlight: Highlight) -> tuple[int, int, int]:
+            a, b, _ = highlight
+            max_range_index = 1
             if b is None:
-                max_range_ind = 0
+                max_range_index = 0
                 b = a
-            return a, max_range_ind, a - b
+            return a, max_range_index, a - b
 
         for line_index in line_range:
-            line_highlights = highlights.get(line_index, []).sort(key=sort_key)
+            highlights.get(line_index, []).sort(key=sort_key)
 
 
 @dataclass