The copy-code-block feature existed but the reporter saw it 'work very sporadically (only a handful of items have it)' and lost after editing. Root causes: it ran ONCE in Template.editor.onRendered with a global document.querySelectorAll('.viewer > pre'), so it decorated only whichever viewers were on the page at that instant and never re-added the button after an edit re-rendered the viewer; and it copied the HTML-escaped innerHTML of the <pre>'s first child node (often the wrong content or undefined) via the deprecated execCommand. Now the decoration lives in Template.viewer.onRendered, so EVERY viewer (card description, checklist item, comment) gets a copy button and it is restored on every re-render; it is de-duplicated (skips a <pre> that already has its button) so re-renders never stack buttons; it copies the RAW code text (pre.textContent) via navigator.clipboard.writeText with an execCommand fallback; and the button title is translated (the existing 'copy-text-to-clipboard' i18n key) instead of a hard-coded English string.
Tests: tests/copyCodeBlock.test.cjs (5 checks) guards that the decoration is in Template.viewer.onRendered scoped to the viewer's <pre>, copies textContent (not innerHTML), de-dups, is translated, and uses the async Clipboard API with a fallback.
Thanks to C0rn3j and xet7 !
Fixes#5149,
