main maincontent padding 5px; margin 0px; overflow auto; width 100; for some explorer browsers to trigger hasLayout adding overflow auto worked, although if I add also width 100, that causes the div (maincontent) to be slightly bigger than it's parent div

The main problem in your code is that you didn't specify display flex on the main container

and add displayflex for blockcolumn The display block property on the parent element prevents flex-grow from having an effect

You have to propagate