Fix for #96968

This is a scoped fix for #96968.

The cause of the issue is the following:

1. Webview must be rendered outside of the main editor DOM. We do this by absolutely positioning them over some element in the DOM.
1. In split views, we try to lay the webview out over an element that has 0 height.
1. Due to my workaround in ea07e9bb, this causes the webview to either not show at all (because it also will have zero height) or partially show

This fix forces the webivew's parent in the split view to have 100%. That actually seems like a reasonable default but I've scoped my fix to just webviews

Fix #96448

Fixes #96503

This workaround no longer seems needed

Fixes #96915

Fixes #96621

Fix #96678

#96308

#96308

Text buffer in CellTextModel and hook up with Working Copy Service

[Notebook] split and join

fixes #96046

fixes #96607