Webpack drops aggregated changes when using Watching.resume()

[vladar]

Bug report

What is the current behavior?

This is an issue with resume/suspend flow of webpack Watching. In an edge case (described below) webpack drops some of the aggregated changes and they are not included in the compilation result.

If the current behavior is a bug, please provide the steps to reproduce.

All of the following conditions must be met to reproduce it:

1. resume is called between changed (invalid) and aggregated watchpack events
2. empty compilation caused by resume is finished before aggregated event fires

I've opened a PR with a failing test to demonstrate it: #12881

It is easier to reproduce this with higher aggregation timeout but we've seen this in Gatsby in the real-world projects with default timeout as well (just harder to reproduce).

What is the expected behavior?

Resumed compilation should include all aggregated changes.

Other relevant information:
webpack version: latest
Node.js version: 12.20.0
Operating System: Windows
Additional tools: Jest? 😄

[alexander-akait]

Fixed by #13399