The fillspace and adjust properties provide totally different behavior for column sizing and weren’t intended for use in one column simultaneously.
So far I can suggest you use the fillspace in the initial config and apply adjustColumn method when it’s necessary.
But please note that dynamic changes for fillspace can be applied only through the column’s config and after refreshing the entire structure: https://webix.com/snippet/c5fae4f2
In case of adjusting to the column header, there’s another option: as the fillspace reflects the minWidth of the column, you can set the minimal width based on the header using getTextSize method: https://webix.com/snippet/3f649018