fix VerticalScroll::move_top for PageUp

Author: xlai89

src/components/utils/scroll_vertical.rs

@@ -38,9 +38,14 @@ impl VerticalScroll {
 			ScrollType::PageDown => old
 				.saturating_add(self.visual_height.get())
 				.saturating_sub(1),
-			ScrollType::PageUp => old
-				.saturating_sub(self.visual_height.get())
-				.saturating_add(1),
+			ScrollType::PageUp => {
+				if old < self.visual_height.get() {
+					0
+				} else {
+					old.saturating_sub(self.visual_height.get())
+						.saturating_add(1)
+				}
+			}
 			ScrollType::Home => 0,
 			ScrollType::End => max,
 		};
@@ -201,4 +206,26 @@ mod tests {
 		scroll.move_area_to_visible(visual_height, 0, 2);
 		assert_eq!(scroll.get_top(), 0);
 	}
+
+	#[test]
+	fn test_scroll_with_pageup_pagedown() {
+		let scroll = VerticalScroll::new();
+		scroll.max_top.set(10);
+		scroll.visual_height.set(8);
+
+		assert!(scroll.move_top(ScrollType::End));
+		assert_eq!(scroll.get_top(), 10);
+
+		assert!(!scroll.move_top(ScrollType::PageDown));
+		assert_eq!(scroll.get_top(), 10);
+
+		assert!(scroll.move_top(ScrollType::PageUp));
+		assert_eq!(scroll.get_top(), 3);
+
+		assert!(scroll.move_top(ScrollType::PageUp));
+		assert_eq!(scroll.get_top(), 0);
+
+		assert!(!scroll.move_top(ScrollType::PageUp));
+		assert_eq!(scroll.get_top(), 0);
+	}
 }