Render initial preview for empty documents

[sdirix]

Improves preview-widget to also render previews for empty documents.

Fixes #8728

Signed-off-by: Stefan Dirix sdirix@eclipsesource.com
Contributed on behalf of STMicroelectronics

What it does

The preview-widget checks whether the content of the underlying text document changed before rerendering its content. In case of empty documents this check prevented the initial rendering of the preview. This is now fixed by distinguishing between empty and non-existing content.

How to test

1. Implement a new PreviewHandler which renders content for empty documents
2. Open preview on empty document
3. Check whether the preview shows the content returned above

You can use this PreviewHandler for testing:

@injectable()
class MyPreviewHandler implements PreviewHandler {
  canHandle(uri: URI) {
    return 1000;
  }
  renderContent() {
    const div = document.createElement("div");
    div.innerText = "my preview";
    return div;
  }
}

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

[vince-fugnitto]

The changes work well for me with the provided example.
When documents are empty, the preview is now correctly rendered.

[vince-fugnitto]

I'll merge tomorrow if there are no objections.

[sdirix]

@vince-fugnitto Thanks for the review!