Solution 1 :

Try to change style using pseudo selector:

option:checked {
width: fit-content

Problem :

I am using HTML Multi select with fixed width but when i am selecting any value which has longer text, the selection does not happens properly and text is not visible after scroll and its not even highlighted.

I have attached the JS Fiddle link for easy replication of the issue.

    <select multiple="true" name="available" size="15" style="width:250px; overflow-x: auto">
      <option value="1">Test 1</option>
      <option value="2">Lorem Ipsum has been the industry's standard dummy text ever since the 1500s</option>

enter image description here