React version: 0.0.0-f77c7b9d7
Browser: Chrome Version 84.0.4147.105 (Official Build) (64-bit)
OS: Ubuntu 18.04.4 LTS

## Steps To Reproduce

1. move mouse over button

Link to code example:

https://codesandbox.io/s/mouseenter-in-reactnext-ibld3?file=/src/Demo.js

## The current behavior

`onMouseEnter` fires twice (sometimes it doesn't).

![video capture of repro steps](https://i.ibb.co/XzdQF68/react-next-mouseenter-twice.gif)

## The expected behavior

It fires only once. Same repro with `react@16.13.1`: https://codesandbox.io/s/mouseenter-in-react16131-9sr7b?file=/src/Demo.js

</svg>
                <image {...expectSVG} />
              </svg>,
            )}
            <p {...expectHTML} />
          </foreignObject>
          <image {...expectSVG} />
        </svg>
        <p {...expectHTML} />
      </div>,
    );
  });
  it('should unwind namespaces on uncaught errors', () => {
    function BrokenRender() {
      throw new Error('Hello');
    }
    expect(() => {
      assertNamespacesMatch(
        <svg {...expectSVG}>
          <BrokenRender />
        </svg>,
      );
    }).toThrow('Hello');
    assertNamespacesMatch(<div {...expectHTML} />);
  });
  it('should unwind namespaces on caught errors', () => {
    function BrokenRender() {
      throw new Error('Hello');
    }
    class ErrorBoundary extends React.Component {