CSS scroll snap

const positions = ["start", "center", "end"];
const inlineDirection = document.getElementById("inline");
const blockDirection = document.getElementById("block");
const stop = document.getElementById("stop");
const snap = document.getElementById("snap");
const all = document.querySelector("article");
const rules = document.styleSheets[0].cssRules;

inlineDirection.addEventListener("change", () => {
  setSSA();
});
blockDirection.addEventListener("change", () => {
  setSSA();
});
stop.addEventListener("change", () => {
  setSST();
});
window.addEventListener("load", () => {
  setSST();
  setSSA();
});
snap.addEventListener("change", () => {
  all.classList.toggle("snapDisabled");
});

function setSSA() {
  rules[0].style.scrollSnapAlign = `${positions[blockDirection.value]} ${
    positions[inlineDirection.value]
  }`;
}

function setSST() {
  if (stop.checked) {
    rules[0].style.scrollSnapStop = "always";
  } else {
    rules[0].style.scrollSnapStop = "normal";
  }
}

<article>
  <ul>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
  </ul>
  <div>
    <fieldset>
      <legend>Change the options</legend>
      <p>
        <label
          ><input
            type="range"
            min="0"
            max="2"
            value="1"
            list="places"
            id="block" />
          block position</label
        >
      </p>
      <p>
        <label>
          <input
            type="range"
            min="0"
            max="2"
            value="1"
            list="places"
            id="inline" />
          inline position
        </label>
      </p>
      <p>
        <label>
          <input type="checkbox" id="stop" />
          Prevent scrolling past boxes
        </label>
      </p>
    </fieldset>

    <p>
      <label><input type="checkbox" id="snap" /> disable snapping</label>
    </p>

    <datalist id="places">
      <option value="0">start</option>
      <option value="1">center</option>
      <option value="2">end</option>
    </datalist>
  </div>
</article>

li {
  /*
  starts with:
      scroll-snap-align: center center;
      scroll-snap-stop: normal (defaults);

  CSS gets changed with JavaScript when you change the controls.
  the following can be set:
      scroll-snap-stop: always | normal;
      scroll-snap-align: start | center | end {2}
        */
}
ul {
  overflow: auto;
  scroll-snap-type: both mandatory;
  overscroll-behavior-x: contain;
}
article.snapDisabled fieldset {
  opacity: 20%;
  pointer-events: none;
}
article.snapDisabled ul {
  scroll-snap-type: initial;
  overscroll-behavior-x: initial;
}

@layer pageSetup {
  article {
    display: flex;
    gap: 2vw;
  }
  div {
    flex: 1;
  }
  ul {
    display: grid;
    gap: 6.25vw;
    padding: 12.5vw;
    box-sizing: border-box;
    border: 1px solid;
    grid-template-columns: repeat(5, 1fr);
    background: conic-gradient(
      at bottom left,
      red 0deg,
      yellow 15deg,
      green 30deg,
      blue 45deg,
      purple 60deg,
      magenta 75deg
    );
    background-attachment: local;
    margin: auto;
    width: 20vw;
    height: 20vw;
  }
  li {
    scroll-snap-align: center;
    height: 12.5vw;
    width: 12.5vw;
    outline: 3px inset;
    list-style-type: none;
    background: white;
    font-family: monospace;
    font-size: 3rem;
    line-height: 12vw;
    text-align: center;
    counter-increment: items 1;
  }
  li::after {
    content: counter(items);
  }
  input {
    vertical-align: bottom;
  }
  p {
    font-family: monospace;
  }
}