widget: when clicking on scrollbar, center on that point

Previously, we'd scroll so the new viewportStart corresponded to the
clicked position. This felt okay if clicking above the current
indicator, but felt jarring when clicking below it. Centering gives a
consistent behavior regardless of the scroll direction.

Signed-off-by: Dominik Honnef <dominik@honnef.co>

widget/list.go

@@ -46,6 +46,20 @@ func (s *Scrollbar) Layout(gtx layout.Context, axis layout.Axis, viewportStart,
 	trackHeight := float32(axis.Convert(gtx.Constraints.Max).X)
 	s.delta = 0
 
+	centerOnClick := func(normalizedPos float32) {
+		// When the user clicks on the scrollbar we center on that point, respecting the limits of the beginning and end
+		// of the scrollbar.
+		//
+		// Centering gives a consistent experience whether the user clicks above or below the indicator.
+		target := normalizedPos - (viewportEnd-viewportStart)/2
+		s.delta += target - viewportStart
+		if s.delta < -viewportStart {
+			s.delta = -viewportStart
+		} else if s.delta > 1-viewportEnd {
+			s.delta = 1 - viewportEnd
+		}
+	}
+
 	// Jump to a click in the track.
 	for _, event := range s.track.Events(gtx) {
 		if event.Type != gesture.TypeClick ||
@@ -61,7 +75,7 @@ func (s *Scrollbar) Layout(gtx layout.Context, axis layout.Axis, viewportStart,
 		// Clicking on the indicator should not jump to that position on the track. The user might've just intended to
 		// drag and changed their mind.
 		if !(normalizedPos >= viewportStart && normalizedPos <= viewportEnd) {
-			s.delta += normalizedPos - viewportStart
+			centerOnClick(normalizedPos)
 		}
 	}
 
@@ -100,7 +114,7 @@ func (s *Scrollbar) Layout(gtx layout.Context, axis layout.Axis, viewportStart,
 					Y: int(event.Position.Y),
 				})
 				normalizedPos := float32(pos.X) / trackHeight
-				s.delta += normalizedPos - viewportStart
+				centerOnClick(normalizedPos)
 			}
 		} else {
 			s.delta += normalizedDragOffset - s.oldDragPos