Material-UI Breakpoint Indicator
When developing responsive applications with material-ui, it's often helpful to be able to easily visualize the current breakpoint.  Enter, <BreakpointIndicator />.
How does it work?
You can simply import the component at the top level of your app, just after material-ui's ThemeProvider (shown below with redux as well):
import React from 'react';
import ReactDOM from 'react-dom';
import CssBaseline from '@material-ui/core/CssBaseline';
import { ThemeProvider } from '@material-ui/core/styles';
import { theme } from './path/to/my/theme';
import { MyApp } from './path/to/MyApp';
import { configureStore } from './store/configureStore';
import { Provider } from 'react-redux';
import { BreakpointIndicator } from 'mui-breakpoint-indicator';
const store = configureStore();
ReactDOM.render(
  <Provider store={store}>
    <ThemeProvider theme={theme}>
      <CSSBaseline />
      <BreakpointIndicator />
      <MyApp />
    </ThemeProvider>
  </Provider>,
  document.querySelector('#root'),
);
How can it be configured?
the position prop [string, defaults to top-center]
The BreakpointIndicator has an optional position prop with the following type:
type Position =
  | 'bottom-center'
  | 'bottom-left'
  | 'bottom-right'
  | 'left-center'
  | 'right-center'
  | 'top-center'
  | 'top-left'
  | 'top-right'
  ;
The default is top-center, but you can configure the default to whatever position you prefer, e.g.:
<BreakpointIndicator position="bottom-right" />
the visible prop [boolean, defaults to true]
A common use case is to use a tool like this conditionally, e.g.:
const debugMode = useSelector(selectDebugMode);
return (
  <div>
    {debugMode && <BreakpointIndicator />}
    <MyApp />
  <div>
)
While you can still do the above, BreakpointIndicator accepts a visible prop (defaults to true) where you can pass a debugMode flag, as above:
const debugMode = useSelector(selectDebugMode);
return (
  <div>
    <BreakpointIndicator debugMode={debugMode} />
    <MyApp />
  <div>
)
The implementation of the visible prop is such that passing visible={false} is a no-op.
How can it be interacted with?
Resize the Viewport
Simply resize the viewport, and watch the indicator change:

Click to Rotate
Clockwise
You can click on the indicator to temporarily rotate it to the next position, clockwise.

Counter-Clockwise
You can ctrl+click the indicator to rotate it counter-clockwise.

NOTE: When you refresh or rerender your app it will revert to whatever the position prop is set to (or the default, top-center if none is set), so be sure to just set the position prop if you want it to always be in a particular area of the screen.
Can I use this if I'm not using Material-UI?
No.  At this time, this component is directly tied to some material-ui underpinnings.