If issues are selected:

• Ranked after the last selected issue

If no issues are selected:

• Ranked before the top visible issue, where "visible" means the section of the Backlog visible in the viewport (including upcoming sprints but not the current sprint)
• If the current sprint is taking up the entire viewport, then the top of the backlog/future sprints is used

If multiple issues are created at once:

• Ranked in order of creation (as normal), all issues appear in the appropriate position based on the above rules.

Filtering of any kind (Quick, Instant, Epic) should not impact on where issues are placed - the rules of visibility still apply.

Testing Notes

Q: What if the view is stale?
A: We will place the issue according to the currently visible issues, so if they are out of date the issue might appear at the wrong place

Q: What if the viewport is huge or very very small?
A: Same result as if the viewport is normally sized.

Q: What if the detail view is closed/open?
A: doesn't affect the logic

Q: What if multiple issues are selected?
A: We'll place the issue after the last selected issue

Q: What if an epic is selected?
A: We currently don't have epic selection

Q: What if a quick/instant filter is applied?
A: See first answer, we'll place it before the first visible issue. In case the selected issue is instant filtered, we still place it after the selected issue

Q: What if your last selected issue is the last issue but it was moved/deleted?
A: moved: it will be ranked according to the new location. deleted: bad luck, it will be ranked last

Q: Can you fool GH by setting the URL without loading it and creating at the new selection?
A: no, GH doesn't look at the url, it looks at the internal state (which might come from the url when you hit return in the address bar). Even if it did, this wouldn't be the end of the world.

Q: What if you don't have schedule permission?
A: We'll silently ignore rank errors, so the issue will end up at the end of the backlog

Q: What if the selection is within an active sprint?
A: The issue would be added at the top of the backlog

Answers provided by Michael R and Nikolay