docstringd and comments

src/textual/_compositor.py

@@ -63,6 +63,7 @@ class MapGeometry(NamedTuple):
         return self.clip.intersection(self.region)
 
 
+# Maps a widget on to its geometry (information that describes its position in the composition)
 CompositorMap: TypeAlias = "dict[Widget, MapGeometry]"
 
 
@@ -171,10 +172,11 @@ class Compositor:
             Iterable[tuple[int, int, int]]: Yields tuples of (Y, X1, X2)
         """
         inline_ranges: dict[int, list[tuple[int, int]]] = {}
+        setdefault = inline_ranges.setdefault
         for region_x, region_y, width, height in regions:
             span = (region_x, region_x + width)
             for y in range(region_y, region_y + height):
-                inline_ranges.setdefault(y, []).append(span)
+                setdefault(y, []).append(span)
 
         for y, ranges in sorted(inline_ranges.items()):
             if len(ranges) == 1:

src/textual/geometry.py

@@ -484,13 +484,13 @@ class Region(NamedTuple):
         )
 
     def intersection(self, region: Region) -> Region:
-        """Get that covers both regions.
+        """Get the overlapping portion of the two regions.
 
         Args:
             region (Region): A region that overlaps this region.
 
         Returns:
-            Region: A new region that fits within ``region``.
+            Region: A new region that covers when the two regions overlap.
         """
         # Unrolled because this method is used a lot
         x1, y1, w1, h1 = self
@@ -511,10 +511,10 @@ class Region(NamedTuple):
         """Get a new region that contains both regions.
 
         Args:
-            region (Region): [description]
+            region (Region): Another region.
 
         Returns:
-            Region: [description]
+            Region: An optimally sized region to cover both regions.
         """
         x1, y1, x2, y2 = self.corners
         ox1, oy1, ox2, oy2 = region.corners