93Implement an accessible focus trap for a dialog

When a modal dialog is open, keyboard focus must stay inside it. Implement createFocusTrap(container, { initialFocus, returnFocus = true, onEscape } = {}) returning { activate(), deactivate() }:

• activate() remembers the element that had focus, then focuses initialFocus (an element), or the first tabbable element in the container, or the container itself (give it tabIndex = -1 if needed).
• While active, Tab on the last tabbable element moves focus to the first, and Shift+Tab on the first moves to the last.
• If focus somehow lands outside the container (a click, a script), move it back inside.
• Escape calls onEscape.
• deactivate() removes every listener and, if returnFocus, focuses the element that had focus before activate().

Tabbable means: a[href], button, input, select, textarea, [tabindex] and [contenteditable] elements that are not disabled, not tabindex="-1", and not hidden (inside [hidden] or with display: none).