Your browser cleared 300 because each level was a distinct document, dodging the same-document recursion guard. So there are two different ceilings to hit.
failure mode
Distinct: every level differs, so nothing trips the guard — it climbs until the tab runs out of memory. This is the crash-to-failure path.
Heads up: on the distinct path, a high cap can consume gigabytes and crash this tab. That crash is the ceiling. Hit Stop any time.
deepest level alive
0
status
idle
target cap
1500
Pick a mode and press Run probe. The counter climbs live; the watchdog widens its patience as frames get deeper (deep frames are timer-throttled, so a fixed timeout would lie).
The number you get is your machine + your browser on this run — RAM, build, and current load all move it. It is not a universal constant.