inside of iframe content. The table width was greater than the iframe width defined via CSS. The iframe scrolling was off, and the table has stretched the iframe to the minimum calculated width of the table. Other side effect: content of the iframe was not completely visible (cut off on the right side).
|--- Available viewport width -------------|
|--- defined and estimated iframe width ---|
|--- table width in the iframe content > than defined iframe width ---|
|--- iframe stretched to the calculated table width ------------------|
Removing the table form iframe content has fixed the problem.