This only happens in Chrome. It works correctly in Firefox and Edge.

It also seems to be a recent regression in our code, I randomly picked a commit from February and this doesn't happen in that version. I'm bisecting now.

Caused by rGVEDc0799b060844: Source mode performance optimizations.

Looks like we actually need the slugs.

Change 518107 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[VisualEditor/VisualEditor@master] Revert "Source mode performance optimizations"

https://gerrit.wikimedia.org/r/518107

Change 518138 had a related patch set uploaded (by Esanders; owner: Esanders):
[VisualEditor/VisualEditor@master] Restore inline slugs in source mode for cursoring

https://gerrit.wikimedia.org/r/518138

Change 518107 abandoned by Esanders:
Revert "Source mode performance optimizations"

Reason:
Replaced with Ic1af43ba2f521246c5cc37126641883250a5a120

https://gerrit.wikimedia.org/r/518107

Change 518138 merged by jenkins-bot:
[VisualEditor/VisualEditor@master] Restore inline slugs in source mode for cursoring

https://gerrit.wikimedia.org/r/518138

Change 518174 had a related patch set uploaded (by Jforrester; owner: Jforrester):
[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (599facff7)

https://gerrit.wikimedia.org/r/518174

Change 518174 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (599facff7)

https://gerrit.wikimedia.org/r/518174