message: Add comment, fix bad math
  This adds a comment to explain the `max-width` and also fixes it actually use
  the right value. (I had grabbed the wrong part of `margin` the first time.)
diff: @@ -92,7 +92,7 @@ limitations under the License.
  overflow: hidden;
  white-space: nowrap;
  text-overflow: ellipsis;
  - max-width: calc(100% - 18px);
  + max-width: calc(100% - 20px); // 100% of parent minus margin and padding
  }
  .mx_Field input:focus + label,
  