have a look

What browser are you using? This doesn't repro for me in Chrome 49 on Mac.
Firefox 45.0
Confirmed this repros on Firefox 45.0 (OSX).
word-break: break-all on public-DraftEditor-content seems to fix it
Adding word-wrap/word-break to the DraftEditor-content fixes this.
Wanna send in a PR?
(oops, @zachberry beat me to it)
Fixed by https://github.com/facebook/draft-js/commit/7d575904b9a8e5cac762a17c5c6ef5689e915993. Thanks @alunyov!
As a note to anyone who lands here debugging this problem, make sure that your Editor component is not inside a flexbox because that can cause this issue to occur even with the fix in 7d57590
I am experiencing this same issue but only when using codeblocks
https://codesandbox.io/s/1oqr4xyy6j
Most helpful comment
As a note to anyone who lands here debugging this problem, make sure that your Editor component is not inside a flexbox because that can cause this issue to occur even with the fix in 7d57590