Fix StableEditorBottomScrollState

[alexdima]

I think this fixes StableEditorBottomScrollState

But the problem is not completely solved because:

• there is a call to revealRangeNearTopIfOutsideViewport somewhere in the view zone show code.
• (when doing "explain this") - when markdown streams in, the view zone grows but nobody calls StableEditorBottomScrollState
• (when doing "document this") - when edits stream in, I think the view zone grows and also there is a call to revealRangeInCenterIfOutsideViewport

I would continue by figuring out where the view zone changes its height and using StableEditorBottomScrollState there.