If you are referring to the <select> element in your code above.
The answer was given in your other thread of the same topic.
Any simple way to add a scrolling action to a drop-down list?
Straight from MDN Styling HTML forms -
The ugly
Some elements simply can’t be styled using CSS. These include all advanced user interface widgets such as range, color, or date controls as well as all the dropdown widgets, including
<select>,<option>,<optgroup>and<datalist>elements. The file picker widget is also known not to be stylable at all. The new<progress>and<meter>elements also fall in this category.
The browser determines when to initiate a scrollbar depending on the amount of options and the direction the dropdown takes with given viewport space.