func (p CollapsiblePainter) PaintHeader(canvas widget.Canvas, s collapsible.HeaderState) {
	if s.Bounds.IsEmpty() {
		return
	}

	// Resolve colors from theme or defaults.
	bgColor, textColor, arrowColor, focusColor := p.resolveHeaderColors(s)

	// Apply interaction state to background.
	bgColor = dtApplyCollapsibleState(bgColor, s.Hovered, s.Pressed)

	// Draw header background (flat, no rounded corners).
	canvas.DrawRect(s.Bounds, bgColor)

	// 1px bottom border separator.
	borderRect := geometry.NewRect(
		s.Bounds.Min.X,
		s.Bounds.Max.Y-dtCollapsibleBorderWidth,
		s.Bounds.Width(),
		dtCollapsibleBorderWidth,
	)
	canvas.DrawRect(borderRect, dtCollapsibleBorderColor(p.Theme))

	// Draw arrow indicator.
	dtDrawCollapsibleArrow(canvas, s.Bounds, arrowColor, s.ArrowProgress)

	// Draw title text.
	if s.Title != "" {
		titleBounds := dtCollapsibleTitleBounds(s.Bounds)
		canvas.DrawText(s.Title, titleBounds, dtCollapsibleFontSize, textColor, true, dtCollapsibleTextAlign)
	}

	// Focus ring (1px DevTools style).
	if s.Focused {
		canvas.StrokeRect(s.Bounds, focusColor, dtCollapsibleFocusRingWidth)
	}
}

type CollapsiblePainter struct {
	Theme *Theme	// nil uses default DevTools dark fallback
}