Injected scripts cause infinite loop at https://treasure21.timotijhof.net/ducks.html

[Krinkle]

Steps to reproduce

1. Firefox 192.2 (44469)
2. Open https://treasure21.timotijhof.net/ducks.html
3. Click "First clue" and type "spg".

Expected behavior

The input element is removed, and a new input field is inserted and focused.

https://github.com/Krinkle/treasure21/blob/f002bff681cd49d763b47d2b1af5e8e6e2095ae4/public_html/src/ducks.js#L64-L77

Actual behavior

Once the new input is focused, the browser freezes.

Device & build information

• Device: iPhone SE2

Notes

The issue does not happen in:

• Safari on macOS.
• Chrome on macOS.
• Firefox on macOS.
• Mobile Safari on iOS.

It appears specific to Firefox on iOS.

I've debugged this over USB in Safari Web Inspector on macOS and noticed that the dirInputNode.focus(); call (reached once the third character "g" has been entered) goes in to a dozen "Extra Scripts" injected by the firefox-ios app, including various focus and focusin handlers.

What I know:

• When reproducing this without inspector attached, the app freezes.
• When reproducing this with inspector attached, the app freezes,
• When reproducing this, and breaking above dirInputNode.focus();, and setting dirInputNode.focus = function() {}, to effectively skip it and any extra scripts listening to focus/focusin, the app does not freeze.
• When reproducing this, and setting various breakpoints in the extra scripts, and stepping in and out of a few, and then resuming, the app does not freeze. Suggesting there's some kind of async, setTimeout, or postMessage logic in Firefox's content scripts creating a race condition where if you run it slow enough, the code doesn't crash.
• When reproducing this, and breaking above dirInputNode.focus();, and then disabling breakpoints and resuming, the app freezes. Suggesting that it isn't due to a race condition in my own code.

┆Issue is synchronized with this Jira Task