flip animation causes layout thrashing

[wartmanm]

Describe the bug

I have a grid with a large number of elements that I'm animating using flip. It looks great, but it causes the page to freeze up for a few seconds. It seems like fix_position() and create_rule() are causing forced reflows every time they run: fix_position() by interleaving reads to getElementBoundingRect() and getComputedStyle().width, and writes to style.position, and create_rule() by interleaving reads to getElementBoundingRect() and adding style rules.

Reproduction

https://svelte.dev/repl/06ac8d7e24264c5998082b168364df7d?version=3.42.4

Logs

https://share.firefox.dev/3l28BC1

System Info

System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i7-4750HQ CPU @ 2.00GHz
    Memory: 64.80 MB / 8.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 16.9.0 - /usr/local/bin/node
    Yarn: 1.22.11 - /usr/local/bin/yarn
    npm: 7.21.1 - /usr/local/bin/npm
    Watchman: 2021.08.30.00 - /usr/local/bin/watchman
  npmPackages:
    rollup: ^2.3.4 => 2.56.3
    svelte: ^3.0.0 => 3.42.4

Severity

annoyance