Thank you to @Pelagic for reporting this issue: Topic:Wk7cqvino2vjv0jz.

This task is about fixing Automatic Topic Subscription's first-run experience such that the pop-up appears "in front of" the page's chrome when creating a new talk page.

Behavior

1. Visit Special:Preferences to ensure the Automatically subscribe to topics setting is enabled
2. Visit a talk page that does not yet exist. E.g. https://patchdemo.wmflabs.org/wikis/2f3d9efbec/w/index.php?title=Talk:Tea&action=edit&redlink=1 .
3. Click the Start a discussion affordance
4. Type some text into the New Discussion Tool's Title and Description fields

Actual

5. ❗️Notice the You have subscribed pop-up appears is obfuscated by the page's chrome and menus

Legacy Vector	New Vector

Expected

5. ✅Notice the You have subscribed pop-up appears atop/in front of the page's chrome and menus