Make child frames 100% width of parent when layout is vertical

Hello ! Please help.
I am making vertically-layouted frame with other frames (like display: flex, -flex-direction: column). I need children always to be 100% of width of parent !
Look at picture, I need grey ones to be full-width (even if there’s not enough content inside them, any way always fill (by width !))

So parentframe, of course, should have container.layoutMode = 'VERTICAL' . But what should I do further ?