Can't drag and drop cells out of viewport

[divyansshhh]

Description

In lab3, it was possible to drag and drop a cell out of the initial viewport because the viewport would scroll. This doesn't happen in lab4. Here's a gif showing the differences in behavior -

Reproduce

shown in gif

Expected behavior

I expect the behavior in lab4 to be the same as lab3

Context

• JupyterLab v4.1.0rc1

[krassowski]

I can reproduce in 4.1.0 but not in 4.0.12, tagging as regression.

[krassowski]

This is another regression introduced by virtual scrollbar PR (#15533) which changed the DOM structure (confirmed as working in 4.1.0a4 but failing in 4.1.0b0). We need to move data-lm-dragscroll="true" to jp-WindowedPanel-outer and add a test because it's the second time it broke.