Adjust selection filling the curve for CTabFolder

When CTabFolder is selected there is a visible gap on top as well as on
the top left where there is a curve (in RCP when theming is disabled).
Also the image was too close to selection highlight which is adjusted
here as well.

Author: ShahzaibIbrahim

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

@@ -1381,8 +1381,44 @@ void drawSelected(int itemIndex, GC gc, Rectangle bounds, int state ) {
 				// draw highlight marker of selected tab
 				if (parent.selectionHighlightBarThickness > 0 && parent.simple) {
 					Color previousColor = gc.getBackground();
-					gc.setBackground(item.getDisplay().getSystemColor(parent.shouldHighlight() ? SWT.COLOR_LIST_SELECTION : SWT.COLOR_WIDGET_DISABLED_FOREGROUND));
-					gc.fillRectangle(x + 1 /* outline */, parent.onBottom ? y + height - 1 - parent.selectionHighlightBarThickness : y + 1, width - 2 /*outline*/, parent.selectionHighlightBarThickness);
+					gc.setBackground(
+							item.getDisplay().getSystemColor(parent.shouldHighlight() ? SWT.COLOR_LIST_SELECTION
+									: SWT.COLOR_WIDGET_DISABLED_FOREGROUND));
+
+					int thickness = parent.selectionHighlightBarThickness;
+
+					if (parent.onBottom) {
+						gc.fillRectangle(x + 1, y + height - 1 - thickness, width - 2, thickness);
+					} else {
+						java.util.List<Integer> pts = new java.util.ArrayList<>();
+
+				        // Left curve
+				        pts.add(x);         pts.add(y + 2);
+				        pts.add(x + 1);     pts.add(y + 1);
+				        pts.add(x + 2);     pts.add(y);
+
+				        // Flat top across most of the tab
+				        pts.add(x + width - 3); pts.add(y);
+
+				        // Right curve
+				        pts.add(x + width - 2); pts.add(y + 1);
+				        pts.add(x + width - 1); pts.add(y + 2);
+
+				        // Now append bottom edge points in reverse order (parallel to top)
+				        for (int i = pts.size() - 2; i >= 0; i -= 2) {
+				            int px = pts.get(i);
+				            int py = pts.get(i + 1) + thickness;
+				            pts.add(px);
+				            pts.add(py);
+				        }
+
+				        // Convert to primitive int[]
+				        int[] polygon = new int[pts.size()];
+				        for (int i = 0; i < polygon.length; ++i) polygon[i] = pts.get(i);
+
+						gc.fillPolygon(polygon);
+					}
+
 					gc.setBackground(previousColor);
 				}