Block Inserter between blocks doesn't show reliably

[maddisondesigns]

Describe the bug
When trying to insert a block between existing blocks, the Add Block icon rarely shows and when it does, it's quite often in the wrong place.

vid: https://share.getcloudapp.com/7KuLEKAd

To reproduce
Steps to reproduce the behavior:

1. Edit existing page
2. Try to add a new block between your existing blocks by hovering the cursor between blocks

Expected behavior
It shouldn't be this difficult to add a new block to your content

Editor version (please complete the following information):
-WordPress version: [5.5]
-Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? [default]
-If the Gutenberg plugin is installed, which version is it? [e.g., 7.6]

Desktop (please complete the following information):
-OS: macOS Catalina 10.15.6

• Browser: Firefox 79.0

See also #24743

[talldan]

Closing as a duplicate of #22801