WritingFlow: Arrow keys occasionally error, do not navigate to next block

[aduth]

Describe the bug

In writing, I have found that sometimes pressing ArrowLeft or ArrowRight does not navigate to the previous or next blocks as I would expect them too. There is also an error logged to the console:

react-dom.165d5c53.js:283 Uncaught TypeError: Cannot read property 'right' of undefined
    at isEdge (dom.js:166)
    at isHorizontalEdge (dom.js:178)
    at WritingFlow.onKeyDown (index.js:24422)
    at HTMLUnknownElement.callCallback (react-dom.165d5c53.js:143)
    at Object.invokeGuardedCallbackDev (react-dom.165d5c53.js:193)
    at invokeGuardedCallback (react-dom.165d5c53.js:250)
    at invokeGuardedCallbackAndCatchFirstError (react-dom.165d5c53.js:264)
    at executeDispatch (react-dom.165d5c53.js:555)
    at executeDispatchesInOrder (react-dom.165d5c53.js:574)
    at executeDispatchesAndRelease (react-dom.165d5c53.js:674)

gutenberg/packages/dom/src/dom.js

Line 166 in cd3baf4

return Math.round( testRect[ side ] ) === Math.round( rangeRect[ side ] );

(last git blame #14462, cc @ellatrix @nosolosw)

To reproduce

Steps to reproduce the behavior:

1. Navigate to Posts > Add New
2. Click the writing prompt
3. Press Enter
4. Press ArrowLeft
5. Press ArrowRight
6. Observe the first block is still selected, an error logged to the console.

Expected behavior

The second block should be selected, and no error should be logged.

Desktop (please complete the following information):

• OS: macOS Mojave 10.14.5 (18F203)
• Browser: Chrome Version 75.0.3770.100 (Official Build) (64-bit)

[ellatrix]

Curious that none of the e2e tests fail. I'll have a look.

[ellatrix]

Looks like, in case of an empty paragraph, the buttons on the side are in the way, and applying a z-index doesn't get them out of the way.