Add ".gantt_noselect" too $root instead of document.body to make grid column resize work in Shadow DOM.
The codebase has different ways of toggling .gantt_noselect class. Please streamline them all to use $root instead of document.body to make this work in the shadow dom.
Examples from codebase:
✅ gantt3.$root.classList.add("gantt_noselect");
👎 ❌ document.body.className += " gantt_noselect";