SyntaxHighlightingIT#highlightContent is flickering

The functional test org.xwiki.test.ui.docker.AllIT$NestedSyntaxHighlightingIT#highlightContent fails intermittently on ci.xwiki.org.

Occurrence

• Seen in https://ci.xwiki.org/job/XWiki/job/xwiki-platform/job/stable-18.4.x/217/testReport/ (commit 89eec5c383f), environment: default configuration of the main job (Jetty standalone, HSQLDB, Firefox).
• History: 1 failure in 20 builds of the stable-18.4.x main job; passes on master, stable-18.8.x and stable-17.10.x.
• Local verification on 89eec5c383f: the whole module passed with the CI Maven options, and the test passed 20/20 when repeated alone.

Error

org.openqa.selenium.StaleElementReferenceException: The element with the reference c05dbaa3-40f9-4054-a91f-3114388bef4f is stale; either its node document is not the active document, or it is no longer connected to the DOM
	at org.xwiki.test.ui.docker.SyntaxHighlightingIT.getLines(SyntaxHighlightingIT.java:99)
	at org.xwiki.test.ui.docker.SyntaxHighlightingIT.highlightContent(SyntaxHighlightingIT.java:88)

Analysis

SyntaxHighlightingIT#getLines finds the pre.CodeMirror-line elements without waiting and then calls getText() on each. CodeMirror replaces those nodes on every re-render (after the input is processed, after the highlighting pass, on refresh), so when a re-render happens between the lookup and the read the element is stale. The assertion right after typing (assertEquals(List.of(CONTENT, ADDED_CONTENT), getLines(driver))) also does not wait for the expected state.

Possible fix

Wait until getLines() returns the expected lines, retrying on StaleElementReferenceException (same for getMacroMarkers).