[win32] Fix collapsed tab stops at fractional zoom levels

bundles/org.eclipse.swt/Eclipse SWT Tests/win32/org/eclipse/swt/graphics/TextLayoutWin32Tests.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2024 Yatta Solutions
+ * Copyright (c) 2024, 2026 Yatta Solutions and others
  *
  * This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License 2.0
@@ -15,16 +15,26 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
+import java.util.ArrayList;
+import java.util.List;
+
+import org.eclipse.swt.*;
 import org.eclipse.swt.internal.*;
 import org.eclipse.swt.widgets.*;
 import org.junit.jupiter.api.*;
 import org.junit.jupiter.api.extension.*;
+import org.junit.jupiter.params.*;
+import org.junit.jupiter.params.provider.*;
 
 @ExtendWith(PlatformSpecificExecutionExtension.class)
 @ExtendWith(WithMonitorSpecificScalingExtension.class)
 class TextLayoutWin32Tests {
 	final static String text = "This is a text for testing.";
+	private static final String MONOSPACED_FONT = "Courier New";
+	private static final int TAB_STOP_TOLERANCE_IN_POINTS = 2;
 
 	@Test
 	public void testGetBoundPublicAPIshouldReturnTheSameValueRegardlessOfZoomLevel() {
@@ -70,4 +80,51 @@ public void testCalculateGetBoundsWithVerticalIndent() {
 		assertEquals(unscaledBounds.height, scaledBounds.height, 1, "The public API for getBounds with vertical indent > 0 should give a similar result for any zoom level");
 	}
 
+	@ParameterizedTest
+	@ValueSource(ints = { 100, 125, 150, 175, 200 })
+	public void testTabAfterSpacesReachesTheNextTabStop(int zoom) {
+		Display display = Display.getDefault();
+		assumeTrue(isFontInstalled(display, MONOSPACED_FONT), MONOSPACED_FONT + " is not installed");
+
+		List<String> violations = new ArrayList<>();
+		for (int fontHeight = 8; fontHeight <= 20; fontHeight++) {
+			Font font = Font.win32_new(new Font(display, MONOSPACED_FONT, fontHeight, SWT.NORMAL), zoom);
+			for (int tabLength : new int[] { 2, 3, 4, 8 }) {
+				String spaces = " ".repeat(tabLength);
+				// StyledText derives its single tab stop from the width of a run of spaces
+				int tabWidth = boundsWidth(display, font, null, spaces);
+				int spacesThenTab = boundsWidth(display, font, new int[] { tabWidth }, spaces + "\t");
+				int twiceTheSpaces = boundsWidth(display, font, new int[] { tabWidth }, spaces + spaces);
+				if (Math.abs(spacesThenTab - twiceTheSpaces) > TAB_STOP_TOLERANCE_IN_POINTS) {
+					violations.add(fontHeight + "pt with tab length " + tabLength + ": width " + spacesThenTab
+							+ " instead of " + twiceTheSpaces);
+				}
+			}
+		}
+
+		assertTrue(violations.isEmpty(), "A tab placed exactly on a tab stop must advance to the next one, but at zoom "
+				+ zoom + "% it did not for " + violations);
+	}
+
+	private static int boundsWidth(Display display, Font font, int[] tabs, String content) {
+		TextLayout layout = new TextLayout(display);
+		try {
+			layout.setFont(font);
+			layout.setTabs(tabs);
+			layout.setText(content);
+			return layout.getBounds().width;
+		} finally {
+			layout.dispose();
+		}
+	}
+
+	private static boolean isFontInstalled(Display display, String name) {
+		for (FontData fontData : display.getFontList(null, true)) {
+			if (name.equalsIgnoreCase(fontData.getName())) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 }

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/TextLayout.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2000, 2022 IBM Corporation and others.
+ * Copyright (c) 2000, 2026 IBM Corporation and others.
  *
  * This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License 2.0
@@ -387,19 +387,25 @@ void computeRuns (GC gc) {
 	for (int i=0; i<allRuns.length - 1; i++) {
 		StyleItem run = allRuns[i];
 		if (tabsInPixels != null && run.tab) {
+			/*
+			 * Whether a stop is still ahead has to be decided in points, the unit setTabs()
+			 * defined it in. In pixels a stop the pen sits exactly on can round to one pixel
+			 * past the pen, collapsing the tab instead of advancing to the next stop.
+			 */
+			int lineWidthInPoints = DPIUtil.pixelToPoint(lineWidth, getZoom(gc));
 			int tabsLength = tabsInPixels.length, j;
 			for (j = 0; j < tabsLength; j++) {
-				if (tabsInPixels[j] > lineWidth) {
+				if (tabs[j] > lineWidthInPoints) {
 					run.width = tabsInPixels[j] - lineWidth;
 					break;
 				}
 			}
 			if (j == tabsLength) {
-				int tabX = tabsInPixels[tabsLength-1];
-				int lastTabWidth = tabsLength > 1 ? tabsInPixels[tabsLength-1] - tabsInPixels[tabsLength-2] : tabsInPixels[0];
-				if (lastTabWidth > 0) {
-					while (tabX <= lineWidth) tabX += lastTabWidth;
-					run.width = tabX - lineWidth;
+				int tabXInPoints = tabs[tabsLength-1];
+				int lastTabWidthInPoints = tabsLength > 1 ? tabs[tabsLength-1] - tabs[tabsLength-2] : tabs[0];
+				if (lastTabWidthInPoints > 0) {
+					while (tabXInPoints <= lineWidthInPoints) tabXInPoints += lastTabWidthInPoints;
+					run.width = DPIUtil.pointToPixel(tabXInPoints, getZoom(gc)) - lineWidth;
 				}
 			}