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You solve this by accessing the property AutoSizeMode of each column. By default, the columns have AutoSizeMode = NotSet. You will need to define at least one column as AutoSizeMode = Fill together with the property FillWeight = 100.
Example:
this.dataGridView1.Columns[1].AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill;
this.dataGridView1.Columns[1].FillWeight = 100;
Hug.

Try to change to
falsethe propertyRowsHeaderVisible, thus:dataGridView1.RowsHeaderVisible = false;– stderr
@stderr as
RowHeaderVisibleare already infalse.. the case would be to find some Autosize for Columnsheader to resize to DGV size.– Maurício Sanches