Block Editor: ⨁ icon and Block toolbar are missing in Microsoft Edge 18

[chriskmnds]

Describe the bug
It has been reported here (props @zdenys) that Microsoft Edge 18 users are not able to see the block inserter button ⨁ when hovering over blocks, or the block toolbar when clicking on them.

We confirmed briefly that this is not a wp-calypso or WPCOM specific issue. It happens on local WP installs running Gutenberg 7.3.0 (may happen in previous versions too).

To reproduce

1. Starting at URL: Any Post/Page with some Blocks added
2. Hover over spots in between various Blocks
3. Select any Block

Expected behavior

• See the ⨁ icon appearing to add a Block in that spot
• See the Block's toolbar above it when any Block is clicked (selected)

Screenshots

Link: https://d.pr/i/lgdogS

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Edge 18

Additional context
Refer to Automattic/wp-calypso#39072 for more reports of the incident from WPCOM users.

[happychait]

A user reported in 18438062-hc

[aduth]

My understanding is the issue may be caused by the introduction of window.DOMRect in a few places in code:

gutenberg/packages/block-editor/src/components/block-list/insertion-point.js

Line 121 in c6b019b

const rect = new window.DOMRect( clientX, clientY, 0, 16 );

gutenberg/packages/components/src/popover/index.js

Line 81 in c6b019b

const rect = new window.DOMRect(

gutenberg/packages/components/src/popover/index.js

Line 316 in c6b019b

anchor = new window.DOMRect(

Internet Explorer and Edge do not support this feature:

https://developer.mozilla.org/en-US/docs/Web/API/DOMRect#Browser_compatibility

A solution is to include a polyfill. #19871 can be used as prior art, with an equivalent upstream Core patch at Trac#49360 as reference for when this code makes its way to WordPress.

The polyfill-library we have used for Node#contains and URL does appear to have a polyfill for DOMRect as well, which is fortunate and the obvious choice:

https://github.com/Financial-Times/polyfill-library/tree/master/polyfills/DOMRect

cc @ellatrix