# Objective
- Add a test case for #14300Fixes#14300
## Solution
`SceneEntityMapper` relies on operations on `Entities` that require
flushing in advance, such as `alloc` and `free`. Previously, it wasn't
calling `world.flush_entities()` itself and relied on its caller having
flushed beforehand. This wasn't an issue before observers and hooks were
released, since entity reservation was happening at expected times. Now
that hooks and observers are a thing, they can introduce a need to
flush.
We have a few options:
* Flush after each observer/hook run
* Flush between each paired observer/hook and operation that requires a
flush
* Flush before operations requiring it
The first option for this case seemed trickier to reason about than I
wanted, since it involved the `BundleInserter` and its
`UnsafeWorldCell`, and the second is generally harder to track down. The
third seemed the most straightforward and conventional, since we can see
a flush occurring at the start of a number of `World` methods.
Therefore, we're letting `SceneEntityMapper` be in charge of upholding
its own invariants and calling `flush_entities` when it's created.
## Testing
Added a new test case modeled after #14300
fcddb54ce5