Fix the focus trap in a modal dialog

An AI built a modal dialog that works well with a mouse. Keyboard users found that focus stayed on the page behind it, Tab wandered out of it, and Escape did nothing. Your team now writes the dialog's focus behavior down as a function and tests it.

An assistant wrote trap_modal_focus(focusable_ids, trigger_id, keys). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• focusable_ids: the ids of the focusable elements inside the dialog, in Tab order. Two elements can share an id. They are still different elements.
• trigger_id: the id of the element that opened the dialog.
• keys: the keys pressed after the dialog opens: "Tab", "ShiftTab", "Escape" or any other key.
• Return where focus is right after the dialog opens, then after each key. The list has one more entry than keys.

Rules the tests check:

• On open, focus goes to the first position in focusable_ids. With no focusable elements, it goes to the dialog itself, written "dialog".
• Tab moves focus to the next position in focusable_ids, and from the last position back to the first. ShiftTab moves to the previous position, and from the first to the last. So focus never leaves the dialog.
• With no focusable elements, Tab and ShiftTab keep focus on "dialog".
• Any other key leaves focus where it is.
• Escape closes the dialog and returns focus to trigger_id. Once the dialog is closed, focus stays on trigger_id for every later key.