Disclaimer: I have read through a ton of almost my use case but not found exactly the same so:
I want to have a grid with a fixed width left sidebar and the right content column should just fill the rest of the window but not more.
I have it working in a "pure" flex box layout but for some weird reason not within a grid.
What happens is that the flex item will just make the entire grid wider than the window and scroll the entire thing.
What I want is that only the flex container scrolls and the headline above stays where it is.
<style media="screen">
  * {
    margin: 0;
    padding: 0;
  }
  body {
    height: 100%;
  }
  sidebar {
    background-color: red;
    display: block;
  }
  main {
    display: grid;
    grid-template-columns: 300px auto;
    width: 100vw;
  }
  content {
    background-color: blue;
    display: block;
  }
  image-container-wrapper {
    display: block;
    overflow: scroll;
  }
  image-container {
    display: flex;
    flex-wrap: nowrap;
    overflow-x: auto;
  }
  my-image {
    display: block;
    flex: 0 0 auto;
    margin-right: 3px;
    min-width: 100px;
    min-height: 100px;
    background-color: grey;
  }
</style>
<header>Header</header>
<main>
  <sidebar>This should be "fixed"
  </sidebar>
  <content>
    <h1>Slider below</h1>
    <image-container-wrapper>
      <image-container>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
        <my-image></my-image>
      </image-container>
    </image-container-wrapper>
  </content>
</main>
<footer></footer>