Scroll on mouse-down (instead of mouse-up) in the empty area of a scrollbar.

Author: fancidev

src/textual/scrollbar.py

@@ -138,8 +138,8 @@ def render_bar(
             start_index, start_bar = divmod(max(0, start), len_bars)
             end_index, end_bar = divmod(max(0, end), len_bars)
 
-            upper = {"@mouse.up": "scroll_up"}
-            lower = {"@mouse.up": "scroll_down"}
+            upper = {"@mouse.down": "scroll_up"}
+            lower = {"@mouse.down": "scroll_down"}
 
             upper_back_segment = Segment(blank, _Style(bgcolor=back, meta=upper))
             lower_back_segment = Segment(blank, _Style(bgcolor=back, meta=lower))