Matches the changes from #20874 to the regular select element. This is really only necessary for IE and Edge near as I can tell. Fixes #20810.
.custom-select {
display: inline-block;
max-width: 100%;
+ $select-border-width: ($border-width * 2);
+ height: calc(#{$input-height} - #{$select-border-width});
padding: $custom-select-padding-y ($custom-select-padding-x + $custom-select-indicator-padding) $custom-select-padding-y $custom-select-padding-x;
padding-right: $custom-select-padding-x \9;
color: $custom-select-color;