I'm building out a UI that requires a fixed position/sticky element at the bottom of the viewport with a width constrained by a main content area. The main content area is optionally flanked by (sibling) left and/or right sidebars with fixed widths, so I'm using Flexbox to build the three column structure with flex-grow: 1 on the main content.

I've learned from @Marc Audet's accepted answer at How can I make a fixed positioned div inherit width of parent? that setting width: inherit on the fixed element is typically how to solve this problem, but it only seems to work when there's a specified width on its parent, which doesn't help me considering I need the main content area to fill the remaining width of the page.

Does anyone have any ideas for getting around this? Check out my Fiddle for the code/example. Any help would be appreciated!

CSS

html {
  box-sizing: border-box;
  font: 400 16px/1.45 'Source Code Pro';
}

*,
*:before,
*:after {
  box-sizing: inherit;
  margin: 0;
  padding: 0;
  border: 0;
  outline: none;
  overflow-x: hidden;
}

body {
  background: #121;
  color: #FEF;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  width: 100vw;
  height: 100vh;
}

.container {
  display: flex;
  color: #fff;
  height: -moz-fit-content;
  height: -webkit-fit-content;
  height: fit-content;
  background: blue;
}

.left {
  background: blue;
  min-height: 100vh;
  min-width: 150px;
  flex-shrink: 0;
}

.middle {
  background: green;
  min-height: 100vh;
  overflow: hidden;
  width: calc(100vw - 400px);
  padding-bottom: 60px;
  flex-grow: 1;
  flex-shrink: 0;
}

.middle .fixed-footer {
  background: orange;
  position: fixed;
  bottom: 0;
  width: 100vw;
  width: inherit;
  padding: 16px 0;
  overflow-x: hidden;
}

.right {
  background: blue;
  min-height: 100vh;
  min-width: 250px;
  flex-shrink: 0;
}

@media screen and (min-width: 640px) {
  html {
    margin-left: calc(100vw - 100%);
    margin-right: 0;
    overflow-y: auto;
  }
}

Added Star Wars ipsum content to demonstrate .middle's vertical flexibility and how .fixed-footer is stationary and is .middle's width.

DEMO