Drag image is rendered off-screen

[MetRonnie]

Description

Since 2.0.0, the "image" that moves with your mouse when you drag & drop is rendered off-screen instead of under your cursor.

Reproduce

1. Create a DockPanel with a widget in it
2. Drag the widget tab around

Expected behavior

An image of the widget's tab moves around with your cursor.

Context

• Operating System and version: Windows 10
• Browser and version: Chrome 114.0.5735.110
• JupyterLab version: N/A
• @lumino version: 2.0.0

I think this was introduced in #502 which replaced https://github.com/jupyterlab/lumino/blob/578b0a3dfc40ff585dc8ce559c8fb75214a8d4ab/packages/dragdrop/src/index.ts#L234-L235

with https://github.com/jupyterlab/lumino/blob/60060576f0ded96fb2f6d9d3c46cc9f9ca5dd080/packages/dragdrop/src/index.ts#L234

and now it seems the fixed-position offset is incorrect. Setting

top: 0;
left: 0;

sort of fixes it but the top left of the image is anchored to the cursor rather than where you dragged from.

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[fcollonval]

Thanks for reporting!

I guess that setting transform-origin is the way to fix this.

cc @krassowski for input

@MetRonnie would you be willing to open a PR?

[MetRonnie]

PR up at #595