[stable-18.4.x] XWIKI-24800: The WYSIWYG editor sometimes doesn't join the realtime collaboration session

[vmassol]

Jira URL

https://jira.xwiki.org/browse/XWIKI-24800

Backport of #6301 (merged on master as fbf84d5) to stable-18.4.x.

Changes

Description

Clean git cherry-pick -x of the master commit — no adaptation was needed, and the result
reproduces the original commit's stat exactly (1 file changed, 25 insertions, 1 deletion).

• Wait for the CKEditor editing mode to be set before deciding whether the realtime collaboration
  session can be joined.

CKEDITOR.plugins.add('xwiki-realtime').init() runs at pluginsLoaded, but editor.mode is only
assigned much later, from within CKEDITOR.editor.prototype.setMode(), once the mode creator
(i.e. the WYSIWYG iframe) has finished loading. The if (editor.mode !== 'wysiwyg') return true;
guard on the initial join is meant to detect the Source mode, but it also matches "the editing mode
is not set yet", so whenever Loader.bootstrap() won the race the editor silently gave up on
joining the realtime session — no exception, no notification, and no later Source -> WYSIWYG switch
for the mode change handlers to recover from.

Clarifications

• stable-18.4.x is affected: the guard was added by XWIKI-23985 (f582505), first released in
  18.1.0, and is present in this branch at
  xwiki-realtime/plugin.js:252.
• stable-17.10.x and stable-16.10.x predate f582505 and are not affected, so they need no
  backport.
• No pom, @since or Java-level adaptation applies — the change is a single JavaScript file and this
  branch targets the same Java version as master (21).

Screenshots & Video

See #6301 — the CI failure artifacts (screenshot and VNC recording of master build 1361) are attached
to XWIKI-24287.

Executed Tests

Module build on this branch, all checks on (Checkstyle, license, Revapi, Spoon):

mvn clean install -B -ntp -Plegacy \
  -pl xwiki-platform-core/xwiki-platform-ckeditor/xwiki-platform-ckeditor-plugins
# BUILD SUCCESS

The full realtime Docker IT suite was run on the master version of this change (#6301):
RealtimeWYSIWYGEditorIT 28/28 and RealtimeWikiEditorIT 2/2, on containerised Tomcat 11 +
PostgreSQL + Firefox. The functional runs for this branch are left to CI.

Expected merging strategy

• Prefers squash: Yes
• Backport on branches: none — this is the backport.

🤖 Generated with Claude Code

[manuelleduc]

A more idiomatic writing. More generally using the async/await keywords is preferred when possible.

Suggested change

	return whenEditingModeIsSet(editor).then(() => editor._realtime.connect());
	await whenEditingModeIsSet(editor)
	return editor._realtime.connect();