It can be reproduced easily.

1. Make Window, layout: 'fit'
2. Make Grid panel inside window as a child.
3. Set a 'locked: true' one of the columns.
4. Resize window

then you can see vertical scroll jumps to the top.

Below is my example fiddle for this bug.

https://fiddle.sencha.com/#view/editor&fiddle/2k86

and there is another bug.

if you're doing this process(resize and scrolling) repeatedly, you can see some empty records.
In Grid panel, there are some blank records view.

I think it's a critical bug, but it's still not fixed.

It's basic and simple function for grid.
I hope it's fixed as soon as possible.