A Resizable/Collapsible Panel

Task: a group of controls (a settings cluster, a "graph" section) should be collapsible down to just a title bar, reclaiming screen space, while still being bordered and styled consistently with the rest of the sim's non-collapsible panels.

AccordionBox is the collapsible panel — it already draws its own bordered background (styled the same way Panel is) and handles the expand/collapse interaction and layout resizing. You reach for a plain Panel only for a group that's never collapsible; reach for AccordionBox directly when collapsibility itself is the requirement, rather than trying to wrap a Panel in some separate show/hide mechanism.

The solution

import { AccordionBox } from 'scenerystack/sun';
import { VBox, Text } from 'scenerystack/scenery';
import { Checkbox } from 'scenerystack/sun';
import { BooleanProperty } from 'scenerystack/axon';
import { Tandem } from 'scenerystack/tandem';

// --- model-owned state the controls act on ---
const gravityEnabledProperty = new BooleanProperty( true );
const frictionEnabledProperty = new BooleanProperty( false );

// --- the collapsible state itself is also a Property, owned externally so
// it can be read/reset alongside the rest of the screen's state ---
const isControlsExpandedProperty = new BooleanProperty( true );

const controlsContent = new VBox( {
  spacing: 8,
  align: 'left',
  children: [
    new Checkbox( gravityEnabledProperty, new Text( 'Gravity' ), { tandem: Tandem.REQUIRED } ),
    new Checkbox( frictionEnabledProperty, new Text( 'Friction' ), { tandem: Tandem.REQUIRED } )
  ]
} );

const controlsAccordionBox = new AccordionBox( controlsContent, {
  titleNode: new Text( 'Controls' ),
  expandedProperty: isControlsExpandedProperty,
  cornerRadius: 8,
  titleAlignX: 'left',
  tandem: Tandem.REQUIRED
} );

Clicking the title bar (or its expand/collapse button) toggles isControlsExpandedProperty, which collapses controlsAccordionBox down to just the title row — controlsContent's children are hidden, not destroyed, so re-expanding shows the same live controls with no rebuilding.

Resetting the collapsed state with the rest of the screen

Because isControlsExpandedProperty is supplied externally (rather than using expandedDefaultValue), it needs to be reset explicitly alongside the model's own Properties — AccordionBox.reset() is a no-op for externally-supplied Properties:

class MyScreenModel {
  public reset(): void {
    gravityEnabledProperty.reset();
    frictionEnabledProperty.reset();
  }
}

// in the ResetAllButton's listener (see The Reset-All Pattern):
resetAllButton = new ResetAllButton( {
  listener: () => {
    model.reset();
    isControlsExpandedProperty.reset(); // view-owned, but still reset with everything else
  }
} );

If you don't need to observe or reset the expanded state from outside, skip the external Property entirely and just pass expandedDefaultValue: false — AccordionBox will create and own its own expandedProperty, and controlsAccordionBox.reset() resets it for you.

Options used here

Option	Effect
titleNode	Node shown in the always-visible title bar
expandedProperty	Externally-owned Property<boolean> driving expanded/collapsed state
expandedDefaultValue	Alternative to expandedProperty — lets AccordionBox own and reset its own Property
cornerRadius / titleAlignX	Visual styling, matching the conventions of a plain Panel

Collapsed width defaults to matching the expanded width

useExpandedBoundsWhenCollapsed defaults to true, so the collapsed title bar stays exactly as wide as the box is when expanded — this keeps a column of accordion boxes (or an accordion box next to a fixed-width sibling panel) visually aligned regardless of expanded state, instead of the collapsed row shrinking to just the title text's width.

Don't nest a Panel inside an AccordionBox's content expecting a second border

AccordionBox already draws the same kind of bordered background Panel does — wrapping controlsContent in its own Panel before passing it to AccordionBox just produces a border-within-a-border. Pass the bare content (a VBox/GridBox of controls) directly, as shown above.