Restore Section title bar background lost after #3808

Section.updateHeaderImage was skipping the title-bar image whenever the
title-bar gradient color matched the title-bar background color. That
condition is too loose: the title-bar background is typically set
distinct from the section body (e.g. #eaeaea on a #ffffff body in the
light theme) and the image was what painted that contrast across the
title-bar area. After the optimization fired, the title bar collapsed
into the section body color and the visual cue was gone.

Tighten the short-circuit to require all three of gradient color,
title-bar background and section body background to be equal before
skipping the image. This preserves the optimization for genuinely flat
sections while restoring the prior look when the title-bar is meant to
contrast with the body.

Fixes #3945

Author: vogella

bundles/org.eclipse.ui.forms/src/org/eclipse/ui/forms/widgets/Section.java

@@ -464,11 +464,12 @@ protected void onPaint(PaintEvent e) {
 	}
 	private void updateHeaderImage(Color bg, Rectangle bounds, int theight, int realtheight) {
 		Color gradient = getTitleBarGradientBackground() != null ? getTitleBarGradientBackground() : getBackground();
-		if (gradient.equals(bg)) {
-			// Flat look: gradient and background are the same, no image needed
+		if (gradient.equals(bg) && bg.equals(getBackground())) {
+			// Section is uniform; the body background already fills everything.
 			super.setBackgroundImage(null);
 			return;
 		}
+		// Image paints the title bar contrast against the section body.
 		Image image = FormImages.getInstance().getSectionGradientImage(gradient, bg, realtheight,
 				theight, marginHeight, getDisplay());
 		super.setBackgroundImage(image);