Google Search Console is saying:
CLS issue: more than 0.25 (desktop)
First detected: 5/27/20
But the page itself is simply a table containing UTF-8 text. It has an external style sheet, but no images, scripts, or forms. The one possibly uncommon thing is that I serve it as XHTML.
Why would this page experience shift?
Strangely, the complaint occurs only for desktop, not for mobile, even though it is a wide table that can be read much better on a large screen.
(Note that I'm not especially concerned about performance, etc. itself; the page is mostly only for my own use. I'm simply curious about how and why layout shift could be detected.)
UPDATE: I've found a related problem, which I've posted as css - HTML vs. XHTML in timing of loading external style sheets - Webmasters Stack Exchange