fix: adjust styling to avoid hiding IntelliSense popups

[erickzhao]

Problem

Currently, some styling leads to the Monaco editor's IntelliSense being hidden if it would overflow the window.

Screenshot:

Fix

I pinpointed the root cause of the problem to .mosaic-preview in our own src/less/mosaic-vendor.less file, which sets overflow: hidden.

Looking at the original vendor styling (link), it seems like they're not setting any overflow anywhere, so it seems to be on our end.

Since this vendor .less file was originally added in PR #14 to improve FPS, I wanted to test if removing the rule would cause a regression in the scrolling FPS:

1. Screen capture with overflow: hidden
2. Screen capture without

There doesn't seem to be any sort of meaningful regression here, and it seems that removing the rule solves the IntelliSense problem (see video 2).

[coveralls]

Coverage remained the same at 97.105% when pulling 25268ae on fix/intellisense-overflow-hidden into f307964 on master.

[felixrieseberg]

🤔 I feel like there was probably a reason we added the hidden, but I can't remember. My fault for not leaving a comment somewhere, so let's get this in.