[GTK3] Remove show_arrow toggle workaround in ToolBar.computeSize

Remove the gtk_toolbar_set_show_arrow toggle in computeSizeInPixels
that was added in 2012 (Bug 46025) to work around a GTK3 sizing bug
where the toolbar reported only the overflow arrow width.

This workaround causes two GTK state transitions per measurement,
each triggering gtk_widget_queue_resize -> gtk_widget_queue_draw ->
gdk_window_invalidate_region. When a SWT.WRAP ToolBar sits inside a
frequently-measured parent (e.g. CTabFolder.setTopRight), this creates
a self-sustaining ~60 Hz repaint loop that burns 10-25% CPU while idle.

The original GTK3 sizing bug dates from GTK 3.4 era. Modern GTK 3.24
computes preferred size correctly without the show_arrow dance.

Also adds Snippet394 to verify ToolBar.computeSize with SWT.WRAP
returns correct dimensions without the workaround.

Fixes #3236

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vogella

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/ToolBar.java

@@ -177,23 +177,7 @@ Point computeSizeInPixels (int wHint, int hHint, boolean changed) {
 	if (wHint != SWT.DEFAULT && wHint < 0) wHint = 0;
 	if (hHint != SWT.DEFAULT && hHint < 0) hHint = 0;
 
-	Point size = null;
-
-	if (GTK.GTK4) {
-		size = computeNativeSize (handle, wHint, hHint, changed);
-	} else {
-		/*
-		 * Feature in GTK. Size of toolbar is calculated incorrectly
-		 * and appears as just the overflow arrow, if the arrow is enabled
-		 * to display. The fix is to disable it before the computation of
-		 * size and enable it if WRAP style is set.
-		 */
-		GTK3.gtk_toolbar_set_show_arrow (handle, false);
-		size = computeNativeSize (handle, wHint, hHint, changed);
-		if ((style & SWT.WRAP) != 0) GTK3.gtk_toolbar_set_show_arrow (handle, true);
-	}
-
-	return size;
+	return computeNativeSize (handle, wHint, hHint, changed);
 }
 
 @Override

examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet394.java

@@ -0,0 +1,92 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Eclipse Foundation and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Eclipse Foundation - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.swt.snippets;
+
+import org.eclipse.swt.*;
+import org.eclipse.swt.graphics.*;
+import org.eclipse.swt.layout.*;
+import org.eclipse.swt.widgets.*;
+
+/**
+ * ToolBar computeSize test: verify whether the GTK3 toolbar sizing bug
+ * (toolbar reports only the overflow arrow width when show_arrow is enabled)
+ * is still present.
+ *
+ * <p>This snippet creates a ToolBar with several items and compares the
+ * computed size with and without the SWT.WRAP style. If the GTK bug is fixed,
+ * both sizes should be similar. If the bug is still present, the WRAP toolbar
+ * will report a much smaller width (just the arrow).
+ *
+ * <p>See https://github.com/eclipse-platform/eclipse.platform.swt/issues/3236
+ *
+ * <p>For a list of all SWT example snippets see
+ * https://eclipse.dev/eclipse/swt/snippets/
+ */
+public class Snippet394 {
+
+public static void main(String[] args) {
+	Display display = new Display();
+	Shell shell = new Shell(display);
+	shell.setText("Snippet 394 - ToolBar computeSize with WRAP");
+	shell.setLayout(new GridLayout(1, false));
+
+	// Create a toolbar WITHOUT SWT.WRAP
+	ToolBar toolBarNoWrap = new ToolBar(shell, SWT.FLAT | SWT.RIGHT);
+	addItems(toolBarNoWrap);
+
+	// Create a toolbar WITH SWT.WRAP
+	ToolBar toolBarWrap = new ToolBar(shell, SWT.FLAT | SWT.RIGHT | SWT.WRAP);
+	addItems(toolBarWrap);
+
+	// Compute sizes
+	Point sizeNoWrap = toolBarNoWrap.computeSize(SWT.DEFAULT, SWT.DEFAULT);
+	Point sizeWrap = toolBarWrap.computeSize(SWT.DEFAULT, SWT.DEFAULT);
+
+	// Display results
+	Label resultLabel = new Label(shell, SWT.WRAP);
+	resultLabel.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, true, false));
+	StringBuilder sb = new StringBuilder();
+	sb.append("ToolBar without WRAP: " + sizeNoWrap.x + " x " + sizeNoWrap.y + "\n");
+	sb.append("ToolBar with    WRAP: " + sizeWrap.x + " x " + sizeWrap.y + "\n\n");
+
+	int widthDiff = Math.abs(sizeNoWrap.x - sizeWrap.x);
+	if (widthDiff > sizeNoWrap.x / 2) {
+		sb.append("RESULT: Large width difference (" + widthDiff + "px). "
+			+ "The GTK sizing bug may still be present.\n"
+			+ "The WRAP toolbar is likely reporting only the arrow width.");
+	} else {
+		sb.append("RESULT: Widths are similar (diff=" + widthDiff + "px). "
+			+ "The GTK sizing bug appears to be fixed.");
+	}
+	resultLabel.setText(sb.toString());
+
+	// Also print to console
+	System.out.println(sb);
+
+	shell.pack();
+	shell.open();
+	while (!shell.isDisposed()) {
+		if (!display.readAndDispatch()) display.sleep();
+	}
+	display.dispose();
+}
+
+private static void addItems(ToolBar toolBar) {
+	for (int i = 0; i < 5; i++) {
+		ToolItem item = new ToolItem(toolBar, SWT.PUSH);
+		item.setText("Action " + (i + 1));
+	}
+}
+
+}