As a version input, I tried it in Firefox 62, and also didn't see any problems.

In T198272#4319867, @Deskana wrote:

This is bad... but, so far, cannot be reproduced.

Here's the steps I took:

1. Open Firefox (60.0.2 (64-bit))
2. Go to https://en.wikipedia.beta.wmflabs.org/wiki/Selenium_Echo_mention_test_0.3370116106683877
3. Click on "Edit" tab, and watch the visual editor load.
4. Click on "Edit source" tab

Both the expected and actual output were seeing the "Switching to source editing" modal, then seeing the editor switch to source mode.

Am I missing something?

No, I missed a step while trying to replicate it.

You need to save an edit first and then from second attempt onwards it won't switch.

Here are the exact steps:

1. Open VE.
2. Make an Edit and Save.
3. Reopen VE.
4. Now click on "Edit Source" tab.

Also, this is not Firefox specific, happening on Chrome too.

Change 442750 had a related patch set uploaded (by DLynch; owner: DLynch):
[mediawiki/extensions/VisualEditor@master] ArticleTarget switchToWikitextSection make sure switch happens

https://gerrit.wikimedia.org/r/442750

QA note: when merged this patch has a chance of regressing T157755, so double-check behavior with single/multi edit tabs on pages with __NEWSECTIONLINK__.

Change 442750 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] DesktopArticleTarget: consolidate edit tab behavior

https://gerrit.wikimedia.org/r/442750

The original issue is fixed. Also checked for regressions with both single and multi tabs by switching between different tabs in different orders and such. No new regressions found.