[BUG] Align to bottom flickering

Describe the bug The items in the list start to flicker when using alignToBottom option and browser zoom feature. When you use browser zoom feature it creates items with floating point height which likely triggers this issue. The item size keeps recalculating and changing which triggers different padding at the top which results in flickering effect.

Reproduction You can reproduce in “align-to-bottom” e2e example with the following patch: https://github.com/EddyTravels/react-virtuoso/commit/90ee4ea73c3b54779912c9058e9002cbefd93e99

To Reproduce Steps to reproduce the behavior:

1. Open “align-to-bottom”
2. Adjust browser zoom level down to 75%
3. The flickering should start

Screenshots

https://user-images.githubusercontent.com/91422/128022776-7abc8767-a9fa-447e-9320-9a8af824bf0e.mov

(I recorded with item height at 30.8 but I was later able to reproduce with 30px height items as well at different zoom level)

Desktop (please complete the following information):

• OS: Mac OS 11.5.1
• Browser Chrome 92

Additional context

I believe this was also the issue in #317 and #268 but was closed due to lack of reproduction example.

[pranas]

It seems the flickering is gone, great find! Thanks for a quick fix and keep up the good work with react-virtuoso.

[tsnevillecom]

How do you align these to the bottom?