feat(split-panel): add toggleIconOpen/toggleIconClosed props for custom toggle icons

[ernst-dev]

Summary

Implements the feature requested in #4059.

Adds toggleIconOpen and toggleIconClosed props to SplitPanelProps that allow consumers to override the built-in open/close icons on the toggle button with any React.ReactNode (typically an SVG icon).

New API

interface SplitPanelProps {
  /**
   * Custom icon for the close/collapse button (panel open state).
   * Replaces the built-in close/angle icon.
   */
  toggleIconOpen?: React.ReactNode;

  /**
   * Custom icon for the open/expand button (panel closed state).
   * Replaces the built-in angle-up / angle-left icon.
   */
  toggleIconClosed?: React.ReactNode;
}

How it works

When a custom icon is provided it is passed as to the underlying and is set to , suppressing the built-in icon. Applies to:

• Close button in (bottom + side positions, open state)
• Inline open button in (bottom position, closed state)
• Side open button in (side position, closed state)

The AppLayout toolbar trigger button (outside SplitPanel) is not affected.

Files changed

File	Change
	/ props
	Thread icons through to close + bottom-open buttons
	Thread through to side open button
	6 unit tests (all passing)
	Dev page with custom SVG icons

Testing

• Build: ✅ passes
• Unit tests: ✅ 6/6 passing (default icons preserved, custom icons rendered, bottom + side positions)
• ESLint: ✅ clean

Related

Closes #4059