Scrolling to heading does not work for un-rendered markdown cells in defer/none mode

[krassowski]

Description

Even after #15386 scrolling to heading does not work for un-renderd markdown cells.

This does not happen every time, but I was able to reproduce this in both 4.0.10 on binder reliably. In particular adding a new cell "fixes" this for a while.

This is a follow up to #14591 (comment)

Reproduce

0. Set windowing mode to defer (default)
1. Open a notebook longer than viewport with two headings
2. See that ToC works by jumping between headings
3. Un-render the markdown cells
4. See that ToC no longer works (even though cells are switched, they are not scrolled to)
5. Add a new markdown cell with a new heading
6. See that ToC works again

Full repro in 4.0.10 from fresh load:

Also in 4.1.0b0

Expected behavior

The ToC works reliably.

Context

• Browser and version: Chrome
• JupyterLab version: 4.0.10 and 4.1.0b0 (and earlier)