Left-side Sticky Element Disappears When Scrolling To The Right

I am trying to create a web page with five regions, three of them are sticky. The sticky feature works fine when the user scrolls down, but the elements which should stick to the l

Solution 1:

You are having an overlfow issue, your body is a block element so its width won't exceed the dimension of your screen which create the issue of left sidebar.

An easy fix is to make the body inline-grid:

body {
  margin: 1rem;
  display: inline-grid;
  grid-template-columns: 3rem auto;
  grid-template-rows: 12vh 2rem auto;
  grid-template-areas: "header header" "topleft topright" "bottomleft bottomright";}

.header {
  position: fixed;
  grid-area: header;
  display: block;
  background: white;
  text-align: center;
  width: 100vw;
}

.topleft {
  grid-area: topleft;
  background-color: #bababa;
  border: solid 1px black;
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  left: 0;
  z-index: 2;
}

.topright {
  grid-area: topright;
  background-color: #c0c0c0;
  border: solid 1px black;
  box-sizing: border-box;
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  z-index: 1;
  white-space: nowrap;
  display: inline-block;
  width: 300vw; /* simulate large horizontal data set */
}

.bottomleft {
  grid-area: bottomleft;
  background-color: #c0c0c0;
  border: solid 1px black;
  box-sizing: border-box;
  position: -webkit-sticky;
  position: sticky;
  left: 0;
  z-index: 1;
  height: 300vh; /* simulate large vertical data set */
}

.bottomright {
  grid-area: bottomright;
  background-color: #cacaca;
  border: solid 1px brown;
  box-sizing: border-box;
  text-align: left;
  display: inline-block;
  height: 300vh; /* simulate large vertical data set */
  width: 300vw; /* simulate large horizontal data set */
}

<div class="header">
This intro area should be visible until scrolled out of view.
</div>
<div class="topleft">
Stay
</div>
<div class="topright">
Top line sticky vertically but not horizontally
</div>
<div class="bottomleft">
<p>1</p>
<p>2</p>
<p>3</p>
<p>4</p>
<p>5</p>
<p>6</p>
<p>7</p>
<p>8</p>
<p>9</p>
</div>
<div class="bottomright">
Large area with cells
</div>

To better illustrate the issue you can add border to the initial code and you will see that the left sidebar will reach this border and then stop:

body {
  margin: 1rem;
  display: grid;
  border:2px solid red;
  grid-template-columns: 3rem auto;
  grid-template-rows: 12vh 2rem auto;
  grid-template-areas: "header header" "topleft topright" "bottomleft bottomright";}

.header {
  position: fixed;
  grid-area: header;
  display: block;
  background: white;
  text-align: center;
  width: 100vw;
}

.topleft {
  grid-area: topleft;
  background-color: #bababa;
  border: solid 1px black;
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  left: 0;
  z-index: 2;
}

.topright {
  grid-area: topright;
  background-color: #c0c0c0;
  border: solid 1px black;
  box-sizing: border-box;
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  z-index: 1;
  white-space: nowrap;
  display: inline-block;
  width: 300vw; /* simulate large horizontal data set */
}

.bottomleft {
  grid-area: bottomleft;
  background-color: #c0c0c0;
  border: solid 1px black;
  box-sizing: border-box;
  position: -webkit-sticky;
  position: sticky;
  left: 0;
  z-index: 1;
  height: 300vh; /* simulate large vertical data set */
}

.bottomright {
  grid-area: bottomright;
  background-color: #cacaca;
  border: solid 1px brown;
  box-sizing: border-box;
  text-align: left;
  display: inline-block;
  height: 300vh; /* simulate large vertical data set */
  width: 300vw; /* simulate large horizontal data set */
}

<div class="header">
This intro area should be visible until scrolled out of view.
</div>
<div class="topleft">
Stay
</div>
<div class="topright">
Top line sticky vertically but not horizontally
</div>
<div class="bottomleft">
<p>1</p>
<p>2</p>
<p>3</p>
<p>4</p>
<p>5</p>
<p>6</p>
<p>7</p>
<p>8</p>
<p>9</p>
</div>
<div class="bottomright">
Large area with cells
</div>

As a side note, it's good to avoid using the body as a container, better rely on your own container to make it easy to add more structure later or to integrate your code elsewhere

body {
 margin:0;
}
.container {
  margin: 1rem;
  display: inline-grid;
  border:2px solid red;
  grid-template-columns: 3rem auto;
  grid-template-rows: 12vh 2rem auto;
  grid-template-areas: "header header" "topleft topright" "bottomleft bottomright";}

.header {
  position: fixed;
  grid-area: header;
  display: block;
  background: white;
  text-align: center;
  width: 100vw;
}

.topleft {
  grid-area: topleft;
  background-color: #bababa;
  border: solid 1px black;
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  left: 0;
  z-index: 2;
}

.topright {
  grid-area: topright;
  background-color: #c0c0c0;
  border: solid 1px black;
  box-sizing: border-box;
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  z-index: 1;
  white-space: nowrap;
  display: inline-block;
  width: 300vw; /* simulate large horizontal data set */
}

.bottomleft {
  grid-area: bottomleft;
  background-color: #c0c0c0;
  border: solid 1px black;
  box-sizing: border-box;
  position: -webkit-sticky;
  position: sticky;
  left: 0;
  z-index: 1;
  height: 300vh; /* simulate large vertical data set */
}

.bottomright {
  grid-area: bottomright;
  background-color: #cacaca;
  border: solid 1px brown;
  box-sizing: border-box;
  text-align: left;
  display: inline-block;
  height: 300vh; /* simulate large vertical data set */
  width: 300vw; /* simulate large horizontal data set */
}

<div class="container">
<div class="header">
This intro area should be visible until scrolled out of view.
</div>
<div class="topleft">
Stay
</div>
<div class="topright">
Top line sticky vertically but not horizontally
</div>
<div class="bottomleft">
<p>1</p>
<p>2</p>
<p>3</p>
<p>4</p>
<p>5</p>
<p>6</p>
<p>7</p>
<p>8</p>
<p>9</p>
</div>
<div class="bottomright">
Large area with cells
</div>
</div>