Follow-up from "Update feature flag UI to support percentage rollout"

The following discussion from !14538 (merged) should be addressed:

• @pslaughter started a discussion: (+1 comment)

  issue: This sounds like a pre-existing issue (so it shouldn't block this MR), but when I move too fast entering flags (not clicking the "Create" button that pops up) my input gets jumped to the bottom 🤷

  20190725_moving_too_fast_in_feature_flags