Use this snippet in "inject script" in advanced tab of webpagetest. When you do later customize waterfall and show custom marks, you will see layout jumps in wpt waterfall.

layout_shifts_in_wpt

let po = new PerformanceObserver((list) => {
	    for (const entry of list.getEntries()) {
	      //console.log(entry);
		  let val = entry.value;
		  window.performance.mark('layout_shift_'+val);
	    }
	  });

	  po.observe({type: 'layout-shift', buffered: true});