fix(compat): handle listener, style and class in legacy functional components

[xanf]

This one was quite tricky to debug 🕙

Basically, when dealing with legacy functional component we need to enable FUNCTIONAL_COMPONENT compatibility flag for @vue/compat

The problem is that we're wrapping this component inside mount.ts into script-setup component, which triggers Vue to render this component in "Vue 3 mode". Usually this is not a problem - each component controls own compat flags using compatConfig but this is not a case for functional components - they do not have own instance, and compat config calculation is instance-based. This resulted into wrong normalization applied insid @vue/compat and result component simply ignoring event handlers inside test. To fix this we explicitly set FUNCTIONAL_COMPONENT flag in our "wrapper component" so Vue could correctly handle this

Original explanation was wrong, now I figured root cause of the issue. When INSTANCE_LISTENERS or INSTANCE_ATTRS_CLASS_STYLE is set - Vue is slicing "props" to props/listeners/attrs combination. The problem is that script setup simply has no way of accessing listeners (for example) if this compat flag is enabled so they are just lost. In order to mitigate this we explicitly set two flags mentioned in atrrsFallthrough.ts to false for our component making sure they will be passed to our component under test in proper way

[lmiller1990]

I have no idea how you figured this out! Nice job. It seems few libraries are actually using the compat/migration build, but opting for re-writes. Glad to see you are making use of this for Vue Boostrap.