Gadget bugs aren't tracked in Bugzilla per community request (though it's going to be worth reviewing that in the light of Phabricator). Should we mark as INVALID?

WikEd is not an extension hence this is unfortunately out of scope for Bugzilla.
Closing as INVALID as this is not a bug in MediaWiki software or its extension, but in an on-wiki script/gadget.

[OT]

though it's going to be worth reviewing that in the light of Phabricator

The issue tracking tool feels rather unrelated; a central hosting for gadgets is rather what I consider to be needed first.

Part of the problem reported on this task is TemplateData's fault, which should be using the default module "jquery.textSelection" for interaction with the textarea instead of these custom methods:
https://github.com/wikimedia/mediawiki-extensions-TemplateData/blob/9e721d24326a81529ea07739dff1657ed1c677f5/modules/ext.templateDataGenerator.target.js#L343-L353
(this is what is done by CodeEditor, CodeMirror and gadgets/users scripts in general)

I think this task can be re-purposed for implementing that, so I'm reopening it.

Change 405668 had a related patch set uploaded (by Esanders; owner: Esanders):
[mediawiki/extensions/TemplateData@master] Use textSelection API to modify textarea

https://gerrit.wikimedia.org/r/405668

Change 405668 merged by jenkins-bot:
[mediawiki/extensions/TemplateData@master] Use textSelection API to modify textarea

https://gerrit.wikimedia.org/r/405668