Flex Layout with fixed position (no scrolling) sidebar

Question

I have a layout with left and right canvas sidebars, enclosing the Main content area in the middle.

The sidebars and main content are flex items, positioned in a flex la

Answer 1

i dont know how do it with flex, but here is a easyer/alternate css remove all that flex... and try to never add padding to a outer div, its easyer in inner items, then you dont need to calculate if there are to many divs

.flexcontainer {
    display: block;
    min-height: 100%;
    align-items: stretch;
}
.flexitem {
    display: flex;
}
#canvas-left {
    background: yellow;
    order: -1;
    left: 0px;
    width: 20%;
    position: fixed;
}
#content {
    position: absolute;
    background: green;
    order: 1;
    width: 60%;
    left: 20%;
}
#content p {
    display: block;
    padding: 1rem;
}
#canvas-right{
    background: blue;
    order: 2;
    right: 0px;
    width: 20%;
    position: fixed;
}