Using Flow $Exact type utility with Properties doesn't work

[frenic]

The $Exact type utility in Flow could be useful to fail when an unknown (possibly misspelled) style property is used. But this fails for some reason when a known vendor property is used.

const css: $Exact<CSS.Properties<*>> = {
  height: '1px',           // <- OK
  MozAppearance: 'button', // <- Error
};

// All branches are incompatible:
//  - Either cannot assign object literal to `css` because property `MozAppearance` is missing in
//   `StandardLonghandProperties` [1] but exists in object literal [2].
//  - Or cannot assign object literal to `css` because property `MozAppearance` is missing in
//   `StandardShorthandProperties` [3] but exists in object literal [2].

This however works:

const css: CSS.Properties<*> & $Shape<CSS.Properties<*>> = {
  height: '1px',           // <- Ok
  MozAppearance: 'button', // <- Ok
  someUnknown: 'abc'       // <- Error
};

All I could find was this bug when $Exact is used along with spread. But it doesn't really cover this problem.

I'm trying to understand what's happening but I'm not that familiar with Flow.

[frenic]

Fixed in #67