How to prevent a sticky table header showing horizontally scrolled content in the border?

Question

I want to prevent the red headers from showing in the blue header's border when scrolling horizontally. I must have a white border in the blue headers.

.container {
  overflow: auto;
  width: 200px;
}

table {
  border-collapse: collapse;
}

th {
  background-color: red;
  border-right: 1px solid white;
  color: white;
  padding: 10px;
  z-index: 1;
}

.sticky {
  background-color: blue;
  position: sticky;
  z-index: 2;
}

.test1 {
  left: 0;
}

.test2 {
  left: 57px;
}

.test3 {
  left: 114px;
}

<div class="container">
  <table>
    <thead>
      <tr>
        <th class="sticky test1">
        Test1
        </th>
        <th class="sticky test2">
        Test2
        </th>
        <th class="sticky test3">
        Test3
        </th>
        <th>
        Test4
        </th>
        <th>
        Test5
        </th>
        <th>
        Test6
        </th>
      </tr>
    </thead>
  </table>
</div>

I want to prevent the red headers from showing in the blue header's border when scrolling horizontally. I must have a white border in the blue headers.

Answer 1

You could remove all borders and use e a CSS pseudo-element on each header cell to fake the border effect...

Also, would suggest you fix the width of your sticky columns, that way you can set the left property a bit easier. I've gone ahead to use the exact width as I was getting tiny shifts (~1px) when using the horizontal scroll.

.container {
    overflow-x: auto;
    width: 200px;
}

table {
    border-collapse: collapse;
}

th {
    background-color: red;
    color: white;
    padding: 10px;
    position: relative;
}

.sticky {
    background-color: blue;
    position: sticky;
    z-index: 1;
}

th::before {
    content: "";
    position: absolute;
    top: 0;
    right: 0;
    width: 2px;
    /* Adjust fake border width here */
    height: 100%;
    background-color: white;
}

.test1 {
    left: 0;
}

.test2 {
    left: 55.86px;
    /* this is the exact width of the column header */
}

.test3 {
    left: calc(55.86px * 2);
    /* made this exact to stop the tiny shifts when scrolling */
}

<div class="container">
    <table>
        <thead>
            <tr>
                <th class="sticky test1">
                    Test1
                </th>
                <th class="sticky test2">
                    Test2
                </th>
                <th class="sticky test3">
                    Test3
                </th>
                <th>
                    Test4
                </th>
                <th>
                    Test5
                </th>
                <th>
                    Test6
                </th>
            </tr>
        </thead>
    </table>
</div>