Fix hidden x-scrollbar

[dpordomingo]

fix #184
supersedes #209
related to #196 (comment)

Superset calculates the height of the filtered table with javascript, using hardcoded heights for tabs [1] and buttons [2], so when there is more than one line of tabs, the filtered table does not fit in the available space, appearing a new outer scroll (as it can be seen in the screenshot below) as described by #184.

With this PR, the filtered table is positioned using absolute coordinates, leaving space for all the components in the panel, no matter how much lines of tabs are opened.

If this PR is approved, the SQL Lab would be like this 👇

instead of how it was in the previous version without branding: v0.3.0 👇

[dpordomingo]

Only tested in Chrome-Linux; could you please try in other supported browsers?

[ricardobaeta]

✨ Thank you!

[carlosms]

To add more context: the extra scrollbar that I reported is there even if there is only one row of tabs, since the hardcoded heights do not match the new css.

Which is a different problem to having more than one row of tabs.

Testing this locally, it fixes both problems.

@dpordomingo should this be contributed upstream? The problem will happen with more than one row of tabs, and they might also prefer to have css sizing instead of a hardcoded value that can be more prone to errors.

[dpordomingo]

Yes, this could be contributed into superset.
Do you agree on merging, and postpone the contribution later?
As soon as it's accepted, I'll apply it here.

[carlosms]

ok.

But let's first test it with firefox too at least.

[dpordomingo]

I'd say that it's also ok in Firefox 👇