No blank line after summary breaks task list / checkbox

Extracted from issue #30182 (closed)

When a the <summary> tag does not have a blank line following it, then task items are not recognized as tasks (which is fine - there should be a blank line). However, the frontend thinks it's a valid task item, so checking any following tasks will try to check the invalid one.

This was discovered in an on-boarding issue.

<details>
<summary>No blank line after summary element breaks my onboarding list</summary>
1. [ ] People Ops: do such and such
</details>

* [ ] Task 9
* [x] Task 10
* [ ] Task 11

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No blank line after summary element breaks my onboarding list

1. [ ] People Ops: do such and such

• Task 9
• Task 10
• Task 11

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This is related to issue Inconsistent real-time checkbox