fix: items in split view are not draggable without separator-border

[mdogadailo]

When you remove separator-border class from split-view, the split-view-view is no longer accept items to drag.

It can be reproduced when editorGroup.border has alpha 00.

[joaomoreno]

@bpasero AFAIK the editor grid implements DND without using the grid widget or spltiview at all. I believe you are the most qualified for a review of this PR.

[bpasero]

@joaomoreno I confirmed this fixes the issue. However, this all seems to be due to the fact that .monaco-split-view2 > .split-view-container > .split-view-view does not set position: relative otherwise. I think the easier fix would be to put that property on independent from the border. I am positioning my DND drop widgets absolute within the grid and as such I need position: relative for this math to work.

@mdogadailo maybe you could simplify this change based on these findings?

[joaomoreno]

@bpasero I moved position: relative to all split-view-view instances and everything is good, the change makes sense.

@mdogadailo Since I was already in the code, I'll just commit the changes directly. Sorry for that. Good investigation though.