Please take a look at this documentation about Column Widths
which will help you get a better understanding of how the Grid Columns are setup.
Pertaining to your requirements, the first section under Important notes states:
The Grid renders separate tables for the header and data area when scrolling is enabled. These tables need to have column widths that are synchronized. To ensure this, configure table-layout to fixed. As a result, it is not possible to have a scrollable Grid with automatic table layout—that is, automatic column widths, which depend on the cell content.
I hope this clears things.
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