How to fix the height of a div so that adding data dynamically to it doesn't change it's height?

Solution 1:

change the font size to 1 rem

.secondArg {
  float: right;
  font-size: 1rem;
  font-weight: 900;
  color: rgb(24, 22, 22)}

Solution 2:

Make the font-size for #screen equal to 1rem
then height to 1rem(if you want screen to be one line high) or 2rem(for two line high)
1rem = :root font-size in pixel.
Since you have added padding(and apparently there are two #screen), add padding height in terms of rem to height as well.
Total screen height= height(in rem) + padding(in rem)
Link for rem to pixel conversion: http://www.standardista.com/px-to-rem-conversion-if-root-font-size-is-16px/.