[EuiPopover] Added will-change via JS to help with fuzzy animation

[cchaos]

Fixes #4521

According to these two links:

• https://developers.google.com/web/fundamentals/design-and-ux/animations/animations-and-performance
• https://developer.mozilla.org/en-US/docs/Web/CSS/will-change

The best way to enhance animations in the browser is with will-change but adding this statically to class in CSS is costly so it's best to add it via JS only on instantiation. Luckily, we already were applying a style prop to the EuiPopover panel only when mounted so it was an obvious place to add this property.

before

Screen.Recording.2021-02-16.at.12.44.32.PM.mp4

after

Screen.Recording.2021-02-16.at.12.45.22.PM.mp4

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• [ ] Props have proper autodocs and playground toggles
• [ ] Added documentation
• [ ] Checked Code Sandbox works for the any docs examples
• Added or updated jest tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4527/

[cchaos]

One oddity that I noticed in Safari is the the contents of the super date picker's popover blinks upon opening. I checked with the published docs though and it's happening there too, so it's not related to this change.

Screen.Recording.2021-02-16.at.12.48.21.PM.mp4

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4527/

[chandlerprall]

Added a question, but this LGTM! I don't see any reason to avoid will-change in these instances.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4527/

[snide]

Tested and checked code. LGTM.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4527/

[cchaos]

Jenkins, test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4527/

[cchaos]

Jenkins, test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4527/