Class attribute overwrites CKEditor widget classes on figure element

Problem/Motivation

When adding a CSS class to a table using Editor Advanced Table, the class attribute replaces the essential CKEditor widget classes on the figure wrapper element instead of being applied to the <table> element. This breaks the visual selection handles and widget functionality in the editing view.

Expected behavior:

  <figure class="table ck-widget ck-widget_with-selection-handle">
    <table class="my-custom-class">

Actual behavior:

  <figure class="my-custom-class">
    <table>

Root Cause

When the CKEditor 5 Table Properties plugin is enabled, tables are wrapped in elements in the editing view. The downcast converter for the tableClass attribute uses mapper.toViewElement(), which returns the figure wrapper instead of the actual table element.

Using writer.setAttribute("class", ...) on the figure completely overwrites its existing classes ("table ck-widget ck-widget_with-selection-handle"), breaking the widget's visual functionality.