The select element overflows because of the long inner content (long tag names) on Special:PendingChanges, which causes the whole layout to overflow with a horizontal scroll bar on small screen sizes and mobile. I have attached a patch that adds max-width: 100%; to the particular select, which for some reason is lacking in Codex, and enabling fields to shrink smaller than the size of their internal content by adding min-width: 0;.
Before patch:
After patch:


