UI Freeze and doesn't repaint when using scroll bars on Linux

I'm using HtmlRenderer.Mono to render HTML via Mono .Net Framework 4.7.1 Winforms.

I've written a small demo app which works fine - it's a simple htmlPanel with event subscriptions for loading/caching images - works great.

When I integrate this demo form into an existing larger app, scrolling freezes the UI the the whole app. There's no crash log or output in console....it's as though something crashes the UI thread, The app is still running, as I can see it's process via top and monitor it's CPU load.

The crash isn't instant either. I can slide the scroller up and down for between 2 and 20 seconds.....at some point it locks.

HtmlRenderer.Mono source code doesn't appear to be anywhere so I can't debug it.

Can anyone help?

Thanks!