I downloaded the 1999-05-25-17-M6 Win32 nightly build. When I start apprunner the smoketest page appears with the standard mozilla.org table of contents side panel. If I resize the apprunner window making it more than 640/650 pixels wide, the side panel collapses to about a quarter of its normal width. If I resize the window back to less than 640/650 pixels wide, the sidebar reappears but a scrollbar starts flashing halfway up the page during resizing. When I stop resizing the scrollbar disappears.

This was fixed by karnaze, checkin 5/26 (and in the downloads builds of 5/27). Some other bugs remain for this page, but the 'collapsing left-hand column' is no more. *** This bug has been marked as a duplicate of 6404 ***

Agreed. This is a duplicate of 6404.