fix for spatial calculation

src/textual/_compositor.py

@@ -450,7 +450,7 @@ class Compositor:
 
                     if visible_only:
                         placements = arrange_result.get_visible_placements(
-                            widget.size.region + widget.scroll_offset
+                            container_size.region + widget.scroll_offset
                         )
                     else:
                         placements = arrange_result.placements

src/textual/_spatial_map.py

@@ -35,13 +35,13 @@ class SpatialMap(Generic[ValueType]):
             Iterable of grid squares (tuple of 2 values).
         """
         x1, y1, width, height = region
-        x2 = x1 + width
-        y2 = y1 + height
+        x2 = x1 + width - 1
+        y2 = y1 + height - 1
         grid_width, grid_height = self._grid_size
 
         return product(
-            range(x1 // grid_width, 1 + x2 // grid_width),
-            range(y1 // grid_height, 1 + y2 // grid_height),
+            range(x1 // grid_width, x2 // grid_width + 1),
+            range(y1 // grid_height, y2 // grid_height + 1),
         )
 
     def insert(

tests/test_spatial_map.py

@@ -1,8 +1,33 @@
+import pytest
+
 from textual._spatial_map import SpatialMap
 from textual.geometry import Region
 
 
-def test_region_to_grid():
-    spatial_map = SpatialMap()
+@pytest.mark.parametrize(
+    "region,grid",
+    [
+        (
+            Region(0, 0, 10, 10),
+            [
+                (0, 0),
+            ],
+        ),
+        (
+            Region(0, 0, 11, 11),
+            [(0, 0), (0, 1), (1, 0), (1, 1)],
+        ),
+        (
+            Region(5, 5, 15, 3),
+            [(0, 0), (1, 0)],
+        ),
+        (
+            Region(5, 5, 2, 15),
+            [(0, 0), (0, 1)],
+        ),
+    ],
+)
+def test_region_to_grid(region, grid):
+    spatial_map = SpatialMap(10, 10)
 
-    assert list(spatial_map._region_to_grid(Region(0, 0, 10, 10))) == [(0, 0)]
+    assert list(spatial_map._region_to_grid(region)) == grid