Improved notebook cell drag images

[jonah-iden]

What it does

This improves the rendering of drag images for notebook cells and aligns to more how they look in vscode.

How to test

Open notebook and try dragging a cell.
Code and mardown cells have different drag image renderers

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

I'm wondering whether we should call NotebookCellListView.dragGhost?.remove() on onDrop as well. Otherwise the element stays in the body indefinitely, right?

Aside from that, looks pretty great. Good idea using the markdown renderer to produce "pseudo" editors during dragging.

[msujew]

I've managed to break the markdown rendering by including ``` in the first line of the cell source:

I think it would be alright to just run replaceAll('```', '') on the lines value. It probably doesn't matter if it had any significance in the input text.

Aside from that, I've noticed that scrolling in the notebook doesn't work while dragging a cell, but that's unrelated to the PR.

[jonah-iden]

@msujew thanks, good catch. fixed it the way you suggested for. Not super happy with that though since in python iirc you use ``` to indicate multi line strings.

The scrolling seems to be general theia problem. Same thing happens in the file explorer and other places as soon as you drag anything. If there is not already an issue for that we should probably reate one

[msujew]

@jonah-iden AFAIK python uses """. Anyway, I found a way to correctly escape everything and pushed it to your branch.

[msujew]

Looks good to me, thanks 👍