Calculate chevron size in CTabFolderRenderer without Device#getDPI()

The chevron font size is currently determined using Device#getDPI().
This value is static across the application lifecyle, always returns the
DPI for the primary monitor at application startup. The font does thus
not adapt properly to other zoom values.

This change replaces the logic to calculate the chevron font size to use
a scaled version of the font used for CTabItem. It also adds a listener
for the zoom change event to trigger redraw.

bundles/org.eclipse.swt/Eclipse SWT Custom Widgets/common/org/eclipse/swt/custom/CTabFolder.java

@@ -341,6 +341,7 @@ void init(int style) {
 			case SWT.Selection:        onSelection(event); break;
 			case SWT.Activate:         onActivate(event); break;
 			case SWT.Deactivate:       onDeactivate(event); break;
+			case SWT.ZoomChanged:	   onZoomChange(event); break;
 		}
 	};
 
@@ -362,14 +363,20 @@ void init(int style) {
 		SWT.Resize,
 		SWT.Traverse,
 		SWT.Activate,
-		SWT.Deactivate
+		SWT.Deactivate,
+		SWT.ZoomChanged
 	};
 	for (int folderEvent : folderEvents) {
 		addListener(folderEvent, listener);
 	}
 
 	initAccessible();
 }
+
+private void onZoomChange(Event event) {
+	update();
+}
+
 void onDeactivate(Event event) {
 	if (!highlightEnabled) {
 		return;

bundles/org.eclipse.swt/Eclipse SWT Custom Widgets/common/org/eclipse/swt/custom/CTabFolderRenderer.java

@@ -102,7 +102,8 @@ public class CTabFolderRenderer {
 	static final int FLAGS = SWT.DRAW_TRANSPARENT | SWT.DRAW_MNEMONIC | SWT.DRAW_DELIMITER;
 	static final String ELLIPSIS = "..."; //$NON-NLS-1$
 	private static final String CHEVRON_ELLIPSIS = "99+"; //$NON-NLS-1$
-	private static final int CHEVRON_FONT_HEIGHT = 10;
+	private static final float CHEVRON_FONT_SIZE_FACTOR = 0.7f;
+	private static final int CHEVRON_BOTTOM_INDENT = 4;
 
 	//Part constants
 	/**
@@ -926,7 +927,7 @@ void drawChevron(GC gc, Rectangle chevronRect, int chevronImageState) {
 		Display display = parent.getDisplay();
 		Font font = getChevronFont(display);
 		int fontHeight = font.getFontData()[0].getHeight();
-		int indent = Math.max(2, (chevronRect.height - fontHeight - 4) /2);
+		int indent = Math.max(2, (chevronRect.height - fontHeight - CHEVRON_BOTTOM_INDENT) /2);
 		int x = chevronRect.x + 2;
 		int y = chevronRect.y + indent;
 		int count = parent.getChevronCount();
@@ -1696,9 +1697,8 @@ Color getFillColor() {
 
 	private Font getChevronFont(Display display) {
 		if (chevronFont == null) {
-			Point dpi = display.getDPI();
-			int fontHeight = 72 * CHEVRON_FONT_HEIGHT / dpi.y;
 			FontData fd = parent.getFont().getFontData()[0];
+			int fontHeight = (int) (fd.height * CHEVRON_FONT_SIZE_FACTOR);
 			fd.setHeight(fontHeight);
 			chevronFont = new Font(display, fd);
 		}