Add flush: pre instead of flush: post breaking change to Vue 3 Migration Guide

[dword-design]

Hey, I just migrated a bigger project to Vue 3 and was wondering why the above mentioned breaking change wasn't part of the migration guide since for me it was one of the biggest changes (or did I oversee it? 🤔). It has a big impact in when watchers are called, especially when the watcher fires requests.

Would be great to have this added to the guide so others can benefit from it!

Here are the necessary links:

• Changelog
• Commit
• Issue

[skirtles-code]

Vue 2 was flush: 'pre' by default, so is Vue 3.

The betas for Vue 3.0 were flush: 'post' by default, which is what was described in the issue you linked above. That was changed to bring Vue 3 into line with Vue 2 in the commit you mentioned. As Vue 2 and Vue 3 are the same there shouldn't be any need to add anything to the migration guide.

If you ran into timing issues migrating an application from Vue 2 to Vue 3 then I think those problems are separate to these flush 'changes'.

[dword-design]

@skirtles-code Seems to be this issue that broke it:
vuejs/core#7030