All these tests should build a reasonable frame tree without asserting.

This text is in a positioned box with scrollbars.
Scroll to see this text. (#1)
This text is in a box with scrollbars.
This text should be visible on the main page. (#1)
This text is in a positioned box with columns, without scrollbars.
This text should be visible on the main page. (#2)
This text is in a positioned box with scrollbars and columns.
Scroll to see this text. (#2)
This text is in a box with scrollbars and columns.
This text should be visible on the main page. (#3)