Visible seams in 9 grid slice

[ThaisRobba]

Describe the bug On at least one browser, the slice seams are visible.

To Reproduce Steps to reproduce the behavior:

1. Go to Slice-9-Grid example
2. Observe seams as the scale changes

Expected behavior They should not be visible, just like in Chrome

Screenshots

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Firefox
• Version: 65.0.2