Steps to repro:

1. Open a diff with markdown changes (e.g. https://github.com/dcwither/react-editable-decorator/pull/4/files?utf8=%E2%9C%93&diff=unified&w=1&short_path=04c6e90#diff-04c6e90faac2675aa89e2176d2eec7d8)
2. Scroll down until the button is sticking to the top of the screen
3. Hover over the button

Expected Behavior:

The tooltip either appears below the button or on top of the toolbar

Actual Behavior:

The tooltip appears underneath the PR toolbar.

System Info:
OS X 10.13.4
Chrome Version 65.0.3325.181 (Official Build) (64-bit)