From c529dcb747a548e5a5ec95518624f51277242927 Mon Sep 17 00:00:00 2001
From: Karl Tauber <karl@jformdesigner.com>
Date: Sun, 27 Aug 2023 14:31:30 +0200
Subject: [PATCH] Smooth Scrolling: - fixed jittery repeating-scrolling with
 PageUp/Down keys when reaching the top/bottom/left/right of the viewport (see
 FlatScrollBarUI.setValueAnimated()) - temporary change viewport scroll mode
 only if it is JViewport.BLIT_SCROLL_MODE - use JViewport.SIMPLE_SCROLL_MODE
 when temporary disabling blitting

---
 .../formdev/flatlaf/ui/FlatScrollBarUI.java   | 20 ++++++++++---------
 .../formdev/flatlaf/ui/FlatScrollPaneUI.java  | 17 ++++++----------
 2 files changed, 17 insertions(+), 20 deletions(-)

diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollBarUI.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollBarUI.java
index 7b042e63..0b61fe7b 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollBarUI.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollBarUI.java
@@ -521,23 +521,25 @@ public class FlatScrollBarUI
 	}
 
 	public void setValueAnimated( int initialValue, int value ) {
+		if( useValueIsAdjusting )
+			scrollbar.setValueIsAdjusting( true );
+
+		// (always) set scrollbar value to initial value
+		scrollbar.setValue( initialValue );
+
 		// do some check if animation already running
 		if( animator != null && animator.isRunning() && targetValue != Integer.MIN_VALUE ) {
-			// ignore requests if animation still running and scroll direction is the same
-			// and new value is within currently running animation
-			// (this may occur when repeat-scrolling via keyboard)
+			// Ignore requests if animation still running and scroll direction is the same
+			// and new value is within currently running animation.
+			// Without this check, repeating-scrolling via keyboard would become
+			// very slow when reaching the top/bottom/left/right of the viewport,
+			// because it would start a new 200ms animation to scroll a few pixels.
 			if( value == targetValue ||
 				(value > startValue && value < targetValue) || // scroll down/right
 				(value < startValue && value > targetValue) )  // scroll up/left
 			  return;
 		}
 
-		if( useValueIsAdjusting )
-			scrollbar.setValueIsAdjusting( true );
-
-		// set scrollbar value to initial value
-		scrollbar.setValue( initialValue );
-
 		startValue = initialValue;
 		targetValue = value;
 
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 3e42cece..2e5ee728 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
@@ -561,21 +561,16 @@ public class FlatScrollPaneUI
 	 */
 	static void runWithoutBlitting( Container scrollPane, Runnable r ) {
 		// prevent the viewport to immediately repaint using blitting
-		JViewport viewport = null;
-		int oldScrollMode = 0;
-		if( scrollPane instanceof JScrollPane ) {
-			viewport = ((JScrollPane) scrollPane).getViewport();
-			if( viewport != null ) {
-				oldScrollMode = viewport.getScrollMode();
-				viewport.setScrollMode( JViewport.BACKINGSTORE_SCROLL_MODE );
-			}
-		}
+		JViewport viewport = (scrollPane instanceof JScrollPane) ? ((JScrollPane)scrollPane).getViewport() : null;
+		boolean isBlitScrollMode = (viewport != null) ? viewport.getScrollMode() == JViewport.BLIT_SCROLL_MODE : false;
+		if( isBlitScrollMode )
+			viewport.setScrollMode( JViewport.SIMPLE_SCROLL_MODE );
 
 		try {
 			r.run();
 		} finally {
-			if( viewport != null )
-				viewport.setScrollMode( oldScrollMode );
+			if( isBlitScrollMode )
+				viewport.setScrollMode( JViewport.BLIT_SCROLL_MODE );
 		}
 	}