[Demo] clicking a toolbar button, the highlight is unexpected

Browser: Safari version 11.1.2 (13605.3.8) macOS version 10.13.6 (also happens in Chrome)

Version: 5.1 FL (also happens in current live demo version 5)

Observed button highlighted after clicking several time

1. click the bold button: either the button being highlighted or the font & font size will be highlighted 
2. click increase indent: only the button itself will be highlighted