Increasing window_size above initial_window_size

In the current state of things, the receiver is expected to release capacity using FlowControl::release_capacity. The doc clearly says that increasing the window size above target window size results in an error.

However, I was not able to find such restriction by looking at the RFC Chapter 6.9 WINDOW_UPDATE. The RFC only says that stream-level window size can be changed using SETTINGS_INITIAL_WINDOW_SIZE frame, and that every modification of this value must be applied to active streams as well. For connection-level capacity, it must be increased using WINDOW_UPDATE frames.

I was wondering if we should expose an API to allow this for stream-level window size as well, or modify the existing FlowControl::release_capacity to allow the value going above the target. I am looking for a way to tweak stream-level window size dynamically for particular stream, without impacting all other streams.

What is your opinion on this?

Start with the FlowControl::release_capacity API and its documentation, then compare its behavior with RFC 7540 Section 6.9 and SETTINGS_INITIAL_WINDOW_SIZE. The issue does not identify implementation files or tests; done would require deciding and specifying whether per-stream dynamic window increases are supported and what API behavior should result.