[EuiPopover] After opening a popover, EuiButtonGroup labels appear with delay in Safari

[mbondyra]

The issue only happens in Safari. I couldn't reproduce it in codesandbox because of problems with running eui in safari on codesandbox so I have this branch that reproduces the issue inside the demo eui page.
master...mbondyra:check_popover

The problem is that after opening the popover, the labels appear much later than the rest of the content:

[cchaos]

Thanks @mbondyra! We've noticed this behavior in the EuiSuperDatePicker (with no good solution yet). We hadn't found another instance of EuiPopover contents that did this, so this is very helpful. I'm going to close though as a duplicate of #4534 and moved your comment there (plus generalizing that issue title).