WebPageTest added a counter for the number of layout shifts that happened before FCP (when there was no visible content to actually shift). This will extract the URLs for pages as well as the timing of the first shift relative to FCP and Start Render.
SELECT url, JSON_EXTRACT(payload, '$._LayoutShiftsBeforePaint.count') AS NumBeforePaint, JSON_EXTRACT(payload, '$._LayoutShifts.time') AS FirstShift, JSON_EXTRACT(payload, '$._firstContentfulPaint') AS FCP, JSON_EXTRACT(payload, '$._render') AS Render FROM `httparchive.pages.2021_02_01_mobile` WHERE cast(JSON_EXTRACT(payload, '$._LayoutShiftsBeforePaint.count') as int64) > 0
For the February 2021 mobile crawl, there were 241k sites that reported a layout shift before content painted. Since both metrics are reported by the browser it should not be possible (though technically non-content layout shifts could happen but kind of defeats the spirit of the metric).
Some were quite substantial and showed most of the page shifting before the first frame. i.e. WebPageTest - Visual Comparison