TabbedPane: fixed scrolling tabs with touchpads and high-resolution mouse wheels

CHANGELOG.md

@@ -40,6 +40,8 @@ FlatLaf Change Log
   `tab`) now respect explicitly set background color.
 - TabbedPane: Fixed `IndexOutOfBoundsException` when using tooltip text on close
   buttons and closing last/rightmost tab. (issue #235)
+- TabbedPane: Fixed scrolling tabs with touchpads and high-resolution mouse
+  wheels.
 - Extras: Added missing export of package
   `com.formdev.flatlaf.extras.components` to Java 9 module descriptor.
 - JIDE Common Layer:

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatTabbedPaneUI.java

@@ -221,6 +221,8 @@ public class FlatTabbedPaneUI
 	private Container leadingComponent;
 	private Container trailingComponent;
 
+	private Dimension scrollBackwardButtonPrefSize;
+
 	private Handler handler;
 	private boolean blockRollover;
 	private boolean rolloverTabClose;
@@ -1870,33 +1872,78 @@ public class FlatTabbedPaneUI
 			lastMouseY = e.getY();
 
 			double preciseWheelRotation = e.getPreciseWheelRotation();
+			boolean isPreciseWheel = (preciseWheelRotation != 0 && preciseWheelRotation != e.getWheelRotation());
 			int amount = (int) (maxTabHeight * preciseWheelRotation);
 
+			// scroll at least one pixel to avoid "hanging"
+			if( amount == 0 ) {
+				if( preciseWheelRotation > 0 )
+					amount = 1;
+				else if( preciseWheelRotation < 0 )
+					amount = -1;
+			}
+
 			// compute new view position
 			Point viewPosition = (targetViewPosition != null)
 				? targetViewPosition
 				: tabViewport.getViewPosition();
 			Dimension viewSize = tabViewport.getViewSize();
+			boolean horizontal = isHorizontalTabPlacement();
 			int x = viewPosition.x;
 			int y = viewPosition.y;
-			int tabPlacement = tabPane.getTabPlacement();
+			if( horizontal )
-			if( tabPlacement == TOP || tabPlacement == BOTTOM ) {
 				x += isLeftToRight() ? amount : -amount;
-				x = Math.min( Math.max( x, 0 ), viewSize.width - tabViewport.getWidth() );
+			else
-			} else {
 				y += amount;
-				y = Math.min( Math.max( y, 0 ), viewSize.height - tabViewport.getHeight() );
+
+			// In case of having scroll buttons on both sides and hiding disabled buttons,
+			// the viewport is moved when the scroll backward button becomes visible
+			// or is hidden. For non-precise wheel scrolling (e.g. mouse wheel on Windows),
+			// this is no problem because the scroll amount is at least a tab-height.
+			// For precise wheel scrolling (e.g. touchpad on Mac), this is a problem
+			// because it is possible to scroll by a fraction of a tab-height.
+			if( isPreciseWheel &&
+				getScrollButtonsPlacement() == BOTH &&
+				getScrollButtonsPolicy() == AS_NEEDED_SINGLE &&
+				(isLeftToRight() || !horizontal) || // scroll buttons are hidden in right-to-left
+				scrollBackwardButtonPrefSize != null )
+			{
+				// special cases for scrolling with touchpad or high-resolution wheel:
+				//   1. if view is at 0/0 and scrolling right/down, then the scroll backward button
+				//      becomes visible, which moves the viewport right/down by the width/height of
+				//      the button --> add button width/height to new view position so that
+				//      tabs seems to stay in place at screen
+				//   2. if scrolling left/up to the beginning, then the scroll backward button
+				//      becomes hidden, which moves the viewport left/up by the width/height of
+				//      the button --> set new view position to 0/0 so that
+				//      tabs seems to stay in place at screen
+				if( horizontal ) {
+					//
+					if( viewPosition.x == 0 && x > 0 )
+						x += scrollBackwardButtonPrefSize.width;
+					else if( amount < 0 && x <= scrollBackwardButtonPrefSize.width )
+						x = 0;
+				} else {
+					if( viewPosition.y == 0 && y > 0 )
+						y += scrollBackwardButtonPrefSize.height;
+					else if( amount < 0 && y <= scrollBackwardButtonPrefSize.height )
+						y = 0;
+				}
 			}
 
+			// limit new view position
+			if( horizontal )
+				x = Math.min( Math.max( x, 0 ), viewSize.width - tabViewport.getWidth() );
+			else
+				y = Math.min( Math.max( y, 0 ), viewSize.height - tabViewport.getHeight() );
+
 			// check whether view position has changed
 			Point newViewPosition = new Point( x, y );
 			if( newViewPosition.equals( viewPosition ) )
 				return;
 
 			// update view position
-			if( preciseWheelRotation != 0 &&
+			if( isPreciseWheel ) {
-				preciseWheelRotation != e.getWheelRotation() )
-			{
 				// do not use animation for precise scrolling (e.g. with trackpad)
 
 				// stop running animation (if any)
@@ -2896,6 +2943,8 @@ public class FlatTabbedPaneUI
 			moreTabsButton.setVisible( moreTabsButtonVisible );
 			backwardButton.setVisible( backwardButtonVisible );
 			forwardButton.setVisible( forwardButtonVisible );
+
+			scrollBackwardButtonPrefSize = backwardButton.getPreferredSize();
 		}
 	}
 }