fix scrollbar thumb height

src/textual/scrollbar.py

@@ -112,10 +112,6 @@ class ScrollBarRender:
 
             thumb_size = window_size / step_size * len_bars
 
-            if thumb_size < len_bars:
-                virtual_size += step_size
-                step_size = virtual_size / size
-
             start = int(position / step_size * len_bars)
             end = start + max(len_bars, ceil(thumb_size))
 

src/textual/widget.py

@@ -2664,6 +2664,7 @@ class Widget(DOMNode):
         self._check_refresh()
 
     def _check_refresh(self) -> None:
+        """Check if a refresh was requested."""
         if self._parent is not None and not self._closing:
             try:
                 screen = self.screen

tests/snapshot_tests/test_snapshots.py

@@ -334,3 +334,9 @@ def test_tabs_invalidate(snap_compare):
         SNAPSHOT_APPS_DIR / "tabs_invalidate.py",
         press=["tab", "right"],
     )
+
+
+def test_scrollbar_thumb_height(snap_compare):
+    assert snap_compare(
+        SNAPSHOT_APPS_DIR / "scrollbar_thumb_height.py",
+    )