Markdown cell doesn't always update when editing in split notebook editor

[roblourens]

• Open a notebook with a markdown cell
• Split it
• Edit a markdown cell on one side, see that it's not updated on the other side in preview mode
• If it works, repeat a couple times

Seems that the right events are being fired, but getText in the viewmodel returns the old text. If I put the preview cell into edit mode, the editor has the right contents.

Any idea what's going on @rebornix? I touched this area recently but I can still repro after reverting my changes.

Pushed a1feacf because I removed that and I think I shouldn't have.

[roblourens]

Now I've really broken it

[rebornix]

I was not able to reproduce this issue with #100545 .