Optimization: throttle invocations of handleDragOver's state updates
Optimization: throttle invocations of handleDragOver's state updates

  - onDragOver gets invoked repeatedly for as long as the user keeps hovering over the element
  - we need to *always* call event.preventDefault, but updates aren't necessary every few milliseconds

stride-nyc/remote_retro,stride-nyc/remote_retro,stride-nyc/remote_retro