Expression: Keep buttons below the input for tall inputs

Summary:
Move the tex buttons when you type tall things into expressions

Test Plan:
Type 1/2+1/2+1/2+1/2+1/2 into an expression box, giving

           1
    ----------------
             1
    2 + -----------
              1
         2 + ---
              2

And see that the buttons are strictly below the input

Auditors: joel