Wait for the cell toolbar items to be rendered the first time before looking for overlap

[brichet]

When the cell toolbar is attached the first time, the items may not be rendered when we try to see if it overlap the cell content.
This make the toolbar to be displayed when it should not (see #15305 (comment))

References

Related to #15305, and more specifically should fix #15305 (comment)

Code changes

• hide the toolbar by default when it is added to the cell, to avoid flickering
• wait for all the items to be rendered, using requestIdleCallback() instead of requestAnimationFrame() before looking for overlap (this should wait for changes in the cell and in the toolbar to be effective).

User-facing changes

Avoid the toolbar to be displayed when it should not.

Backwards-incompatible changes

None

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[JasonWeill]

I see a slight (~1 frame) period after focusing a cell before the toolbar appears, but this should be OK in day-to-day use. Good work!

[krassowski]

I see a slight (~1 frame) period after focusing a cell before the toolbar appears

The delay is slightly distracting annoying when clicking on the editor. That said, when just adding cells, I think the delay is less annoying than the previous behaviour of rendering one button at a time (captured with x6 CPU throttling):

Before	After

[krassowski]

Maybe there should be an opacity-based transition so that they appear smoothly?

Edit: I opened #16310 to discuss