Scale Tree.INCREMENT by zoom level instead of using fixed pixels

[ShahzaibIbrahim]

The Tree.INCREMENT constant defines how many pixels the horizontal scrollbar jumps when clicking the left or right buttons. Previously, this was a fixed pixel value, which made scrolling feel too small on high-DPI monitors.

This change redefines INCREMENT in points and converts it to pixels based on the current zoom level, ensuring consistent scrolling increments across different display scales.

How to Test

• Run the following snippet with 250% zoom

import org.eclipse.swt.*;
import org.eclipse.swt.graphics.*;
import org.eclipse.swt.layout.*;
import org.eclipse.swt.widgets.*;

public class TestHorizontalScrollWidth {
    public static void main (String [] args) {
        int rowCount = 40;
        int columnCount = 15;
		final Display display = new Display();
		System.setProperty("swt.autoScale", "quarter");
		System.setProperty("swt.autoScale.updateOnRuntime", "true");
        Shell shell = new Shell (display);
        shell.setText("Snippet 234 - Tree (Single)");
        shell.setLayout(new FillLayout());

        final Tree tree = new Tree(shell, SWT.MULTI | SWT.H_SCROLL | SWT.V_SCROLL | SWT.FULL_SELECTION);
        tree.setHeaderVisible(true);
        tree.setLinesVisible(true);

        // Create columns
        TreeColumn column1 = new TreeColumn(tree, SWT.NONE);
        column1.setText("Name");
        column1.setWidth(150);
        for (int i = 0; i < columnCount; i++) {
            TreeColumn column = new TreeColumn(tree, SWT.NONE);
            column.setText("Value " + i);
            column.setWidth(200);
        }

        // Create rows
        for (int i = 0; i < rowCount; i++) {
            TreeItem item = new TreeItem(tree, SWT.NONE);
            item.setText(0, "item " + i);
            for (int j = 0; j < columnCount; j++) {
                item.setText(j + 1, "Item " + i + " value @ " + j);
            }
        }

        // Draw consistent selection background (Windows: keep highlight even without focus)
        Listener eraseListener = event -> {
            event.detail &= ~SWT.HOT;
            if ((event.detail & SWT.SELECTED) != 0) {
                GC gc = event.gc;
                Rectangle rect = event.getBounds();
                gc.setForeground(display.getSystemColor(SWT.COLOR_LIST_SELECTION_TEXT));
                gc.setBackground(display.getSystemColor(SWT.COLOR_LIST_SELECTION));
                gc.fillRectangle(rect);
                event.detail &= ~SWT.SELECTED;
            }
        };
        tree.addListener(SWT.EraseItem, eraseListener);

        shell.setSize(800, 400);
        shell.open ();
        while (!shell.isDisposed ()) {
            if (!display.readAndDispatch ()) display.sleep ();
        }
        display.dispose ();
    }
}

Result ( Please click on the gifs below to see it running )

Before :

**After : **

Maybe it's harder to notice but now scrolling using button jumps 10 pixels ahead instead of 5 due to 200% zoom (double the size) and hence it jumps now as per zoom level.

[github-actions]

Test Results

   546 files  ±0     546 suites  ±0   36m 44s ⏱️ - 2m 25s
 4 426 tests ±0   4 409 ✅ ±0   17 💤 ±0  0 ❌ ±0 
16 750 runs  ±0  16 623 ✅ ±0  127 💤 ±0  0 ❌ ±0 

Results for commit a11659d. ± Comparison against base commit 921c544.

♻️ This comment has been updated with latest results.

[akoch-yatta]

Change is solid and does what is expected to me. It is now more consistent for different zooms.