Fix context menu flash in Firefox

[Tbaut]

I was still having issues despite my earlier suggestions, so I refactored a tiny bit and this time I can't reproduce any more.

This PR gets rid of the open/isOpen props and css. Unify the anchorPosition and contextMenuPosition which are the same.

How I could reproduce on Firefox before:

• refresh the page (with cache clean with ctrl+R)
• scroll
• right-click -> the first time it was flashing consistently.

[tanmoyAtb]

yup, I don't see any flashes or lag in the menu opening anymore.
I have a feeling this is because of the transition and slight delay in css picking up the position.

This looks to the point, going to merge it in.