We start a mutable borrow here:

We call a method that can transitively GC here:

When a GC occurs, we panic while trying to trace the self.store_handles member because the mutable borrow is still active.

This can be reproduced with ./mach test-wpt /IndexedDB/nested-cloning-large.any.html --pref js_mem_gc_zeal_level=2 --pref js_mem_gc_zeal_frequency=1 --timeout-multiplier=20 in a --debug-mozjs build.