func (p DefaultPainter) PaintZoneBorder(canvas widget.Canvas, borderRect geometry.Rect, _ Zone) {
	if borderRect.IsEmpty() {
		return
	}
	canvas.DrawRect(borderRect, defaultZoneBorderColor)
}

func (p DefaultPainter) PaintZoneTabs(canvas widget.Canvas, ps ZoneTabsPaintState) {
	if ps.TabBarBounds.IsEmpty() {
		return
	}

	hasScheme := ps.ColorScheme != (ZoneColorScheme{})

	// Tab bar background.
	bg := defaultTabBarBgColor
	if hasScheme {
		bg = ps.ColorScheme.TabBarBackground
	}
	canvas.DrawRect(ps.TabBarBounds, bg)

	// Draw each tab.
	for i := range ps.Tabs {
		ts := &ps.Tabs[i]
		paintDefaultZoneTab(canvas, ts, hasScheme, ps.ColorScheme)
	}

	// Draw indicator under active tab.
	if ps.ActiveIdx >= 0 && ps.ActiveIdx < len(ps.Tabs) {
		active := &ps.Tabs[ps.ActiveIdx]
		indicatorBounds := geometry.NewRect(
			active.Bounds.Min.X,
			active.Bounds.Max.Y-zoneIndicatorHeight,
			active.Bounds.Width(),
			zoneIndicatorHeight,
		)
		indicatorColor := defaultZoneIndicatorColor
		if hasScheme {
			indicatorColor = ps.ColorScheme.ActiveTabBackground
		}
		canvas.DrawRect(indicatorBounds, indicatorColor)
	}
}

type ZoneTabsPaintState struct {
	// Zone identifies which zone this tab bar belongs to.
	Zone	Zone

	// TabBarBounds is the tab bar area.
	TabBarBounds	geometry.Rect

	// Tabs contains the state of each tab in the zone.
	Tabs	[]ZoneTabState

	// ActiveIdx is the index of the currently active tab.
	ActiveIdx	int

	// ColorScheme provides theme-derived colors for painting.
	// Zero value means the painter should use its built-in defaults.
	ColorScheme	ZoneColorScheme
}

type ZoneTabState struct {
	// Title is the panel's display text.
	Title	string

	// Bounds is the tab's clickable area within the tab bar.
	Bounds	geometry.Rect

	// Active is true when this tab is the currently selected tab.
	Active	bool

	// Hovered is true when the mouse is over this tab.
	Hovered	bool

	// Closeable is true when this tab shows a close button.
	Closeable	bool

	// CloseButtonBounds is the clickable area for the close button.
	// Empty if not closeable.
	CloseButtonBounds	geometry.Rect
}

type ZoneColorScheme struct {
	TabBarBackground	widget.Color
	ActiveTabText		widget.Color
	InactiveTabText		widget.Color
	ActiveTabBackground	widget.Color
	HoverBackground		widget.Color
	Border			widget.Color
	CloseButton		widget.Color
}

type DefaultPainter struct{}

type Painter interface {
	// PaintZoneTabs renders the tab header bar for a zone group.
	PaintZoneTabs(canvas widget.Canvas, state ZoneTabsPaintState)

	// PaintZoneBorder renders the border between a zone and the center.
	PaintZoneBorder(canvas widget.Canvas, borderRect geometry.Rect, zone Zone)
}