Investigate better UX for previewing things

People use previewHtml for building UI which does’t work well. We should investigate different options~

We will tackle the problem from the other side: https://github.com/Microsoft/vscode/issues/22068#issuecomment-293903020

[smelukov]

Maybe the better solution is provided a possibility to build own UI inside a tab through DOM-access (like in Atom). Something like DOMContentProvider or like that… Or access to DOM of previewHtml

For example: if I want to insert an iframe inside a tab and get access to its contentWindow-property, then previewHtml is useless for this task. But if I’ll have access to DOM of a tab or to DOM of rendered HTML, then this task will have a simple solution.

[bpasero]

With https://github.com/microsoft/vscode/issues/132651 it is possible to split an editor into 2 in the same group:

While what is pushed now is limiting to opening the same editor side by side, https://github.com/microsoft/vscode/issues/36700 would allow this for any 2 editors.

There is nothing technical blocking us from simply putting a text file to the left and a preview editor to the right as this hack shows:

But there are some questions:

• how would an extension API look like for markdown to open a preview like that
• the preview editor is not syncing with the text document, so whatever magic we do there needs to expand to side-by-side-editors which probably needs extension API too
• how would a user be able to trigger this UI, should it be another action in the editor toolbar for previewing or even be the default?