Missing the odd horizontal divider on frozen columns when zoom is at various levels

[bertbarabas]

Describe the bug

To Reproduce

1. Go to https://adazzle.github.io/react-data-grid/#/CommonFeatures
2. Zoom to 175%
3. Scroll around and see the odd missing horizontal border
4. If it doesn't happen try other zoom levels like 125, 150 etc.

This only seems to happen on frozen columns so in the example web site it will only happen on the first two columns.