class MouseSourceV2

A push-based reader protocol for a client to receive mouse pointer events.

Streaming begins automatically on connection. Scenic dispatches mouse

events in hover fragments, starting with `ENTERED` and ending with `EXITED`

when the cursor leaves a view.

Flow control and acknowledgment rules:

- The server will not push more than

`MOUSE_SOURCE_V2_MAX_UNACKNOWLEDGED_EVENTS` (or a product-configured

maximum) events without receiving an acknowledgment from the client.

The server (Scenic) will throttle event delivery when unacknowledged

events in flight reach this limit.

- Clients should send acknowledgments periodically (e.g. before reaching

the limit, such as within 20 remaining credits) to avoid being throttled.

- No head-of-line blocking: Because channels and credit limits are

independent per-view, a slow client that fails to acknowledge `EXITED`

events only blocks its own channel. Scenic can immediately start pushing

events to the newly hovered client on its own channel.

The position of a pointer event is defined in the context of a viewport,

situated in the view, identical to [`MouseSource`].