Split handle dragging does not work with embedded frames

[tsmaeder]

Description

When I embed an iframe inside a widget and put that widget inside either a SplitPanel or DockPanel, dragging the split handles to change the widget sizes does not work anymore as soon as you drag the handle quickly over the embedded frame. I've created a modified dock panel example to show the issue (attached below)

Reproduce

1. yarn & yarn build the attached example
2. Open index.html
3. Observe: you see a modified dock panel example with the biggest "squares" being blue and red
4. Very slowly drag the split handle between the red and blue squares
5. Observe: the dragging works and resizes the left and right sides
6. Now drag the handle quickly over the red or right square
7. Observe: the left and right sides of the split are no longer resized.

Expected behavior

The resizing works like it does in the original example

Context

Windows 11, Firefox

[tsmaeder]

Here's the reproducer example:

example-dockpanel.zip

[tsmaeder]

And here's a video:

2025-05-28.10-54-35.mp4

[tsmaeder]

I did a workaround for the problem in SplitPanel for Eclipse Theia in this PR eclipse-theia/theia#15643

[krassowski]

Feel welcome to contribute a patch directly to lumino, I can cut a bugfix release with it once ready.