[RaphaelFakhri]

Fixes #385

Problem

After you add or remove a section inside an mj-wrapper, changing any wrapper property (for example, the background color) removes every child section from the canvas. The components stay in the model, so the export is correct, but the editor shows an empty wrapper.

Cause

The wrapper listens to add remove on its components and calls renderChildren() directly. renderChildren() moves the child elements into the container and leaves this.childNodes pointing at the now-empty childNodes list of the children view. render() refreshes this.childNodes after renderChildren(), but the listener does not. The next rerender() (triggered by change:attributes) replaces the wrapper HTML and moves the nodes in this.childNodes into the new container, which is an empty list.

Fix

After the listener renders the children, store the container childNodes, the same way render() does.

Tests

tests/specs/wrapper.test.ts covers both the remove and the add case: it changes a wrapper attribute and checks that the remaining section elements are still rendered inside the wrapper. Both tests fail without the fix and pass with it. The full suite (36 tests), npm run lint, and npm run typecheck pass.