[DropdownMenu] first item is sometimes selected when opening menu

[benoitgrelard]

Bug report

Current Behavior

When using side === 'bottom' and sideOffset === 0 and no padding anywhere (basically if the first item is flush with the trigger), the first item sometimes gets selected after clicking on the trigger towards the bottom of it.

Expected behavior

It shouldn't get selected.

Reproducible example

https://codesandbox.io/s/mystifying-saha-p031u?file=/src/App.js

Suggested solution

We run our onSelect on mouseup in order to be able to open and select in one fell swoop like MacOS menu/select.
It seems that perhaps the menu shows up before it's completely in place or there's some kind of timing issue.

Additional context

This issue was initially brought to our attention here: radix-ui/design-system#305