Bug: MouseEnter does not fire when coming _from_ disabled elements

<!-- Please provide a clear and concise description of what the bug is. Include screenshots if needed. Please test using the latest version of the relevant React packages to make sure your issue has not already been fixed. --> React version: 16.13.1 ## Steps To Reproduce https://codesandbox.io/s/eager-euler-c72fw?file=/src/App.js 1. move from a disabled element to an element with a `mouseenter` listener. 2. `mouseenter` does not fire Link to code example: https://codesandbox.io/s/eager-euler-c72fw?file=/src/App.js ## The current behavior `mouseenter` does not fire because React calculates mouseenter/leave based on the relative element of the opposing event. e.g. `mouseenter` calculated from the `mouseout` on the button. Browsers do not fire pointer events on disabled elements ## The expected behavior `mouseenter` should fire Previous issue: https://github.com/facebook/react/issues/4251