I was able to replicate the issue and in-house with 10 columns. I could see the focus was at the extreme right side. Usually Focus is automatically set to input elements but in this case it is setting to the close icon that is the reason the scroll is moving to the right.
I could see the focus at the left side by doing the below steps.
1. Do a save-As of "pyReportEditorHeader" Section of Apply to class "Code-Pega-List " into your application ruleset.
2. You will be seeing the below marked Close Icon.
3. Change it to close button as below. save the rule.
After these changes, I could see the focus at the left.