From 5cdef5409b29810e63d2ea36b897e0b267977cf6 Mon Sep 17 00:00:00 2001
From: Karl Tauber <karl@jformdesigner.com>
Date: Fri, 25 Aug 2023 15:24:28 +0200
Subject: [PATCH] Smooth Scrolling: fixed jittery scrolling with trackpad or
 Magic Mouse (if smooth scrolling is enabled)

---
 .../formdev/flatlaf/ui/FlatScrollPaneUI.java  | 27 +++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollPaneUI.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollPaneUI.java
index d4b67c9b..3e42cece 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollPaneUI.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollPaneUI.java
@@ -144,8 +144,7 @@ public class FlatScrollPaneUI
 				scrollpane.isWheelScrollingEnabled() )
 			{
 				if( e.getScrollType() == MouseWheelEvent.WHEEL_UNIT_SCROLL &&
-					e.getPreciseWheelRotation() != 0 &&
-					e.getPreciseWheelRotation() != e.getWheelRotation() )
+					isPreciseWheelEvent( e ) )
 				{
 					// precise scrolling
 					mouseWheelMovedPrecise( e );
@@ -179,6 +178,30 @@ public class FlatScrollPaneUI
 		return UIManager.getBoolean( "ScrollPane.smoothScrolling" );
 	}
 
+	private long lastPreciseWheelWhen;
+
+	private boolean isPreciseWheelEvent( MouseWheelEvent e ) {
+		double preciseWheelRotation = e.getPreciseWheelRotation();
+		if( preciseWheelRotation != 0 && preciseWheelRotation != e.getWheelRotation() ) {
+			// precise wheel event
+			lastPreciseWheelWhen = e.getWhen();
+			return true;
+		}
+
+		// If a non-precise wheel event occurs shortly after a precise wheel event,
+		// then it is probably still a precise wheel but the precise value
+		// is by chance an integer value (e.g. 1.0 or 2.0).
+		// Not handling this special case, would start an animation for smooth scrolling,
+		// which would be interrupted soon when the next precise wheel event occurs.
+		// This would result in jittery scrolling. E.g. on a MacBook using Trackpad or Magic Mouse.
+		if( e.getWhen() - lastPreciseWheelWhen < 1000 )
+			return true;
+
+		// non-precise wheel event
+		lastPreciseWheelWhen = 0;
+		return false;
+	}
+
 	private void mouseWheelMovedPrecise( MouseWheelEvent e ) {
 		// return if there is no viewport
 		JViewport viewport = scrollpane.getViewport();