Dragging connections results in wrong connection

[LeonStaufer]

Describe the bug

When dragging one of the blue connection corners to that of another element, the resulting connection is not between those two connection corners but instead between two seemingly random ones.

To Reproduce

1. Create two elements (e.g. class)
2. Start dragging from one of the blue connection corners
3. Let go at another blue connection corner
4. Often times, the resulting connection is different than expected

Expected behavior

The connection to consistently match the one provided when dragging.

Screenshots

Environment

 - OS: elementary OS 5.1.4 Hera, Built on Ubuntu 18.04.4 LTS, Linux 4.15.0-99-generic
 - Browser Firefox 75.0

Output in Web Console

none

[TobiasPr]

@LeonStaufer Please note that firefox is not fully supported. Please use chrome for now as a fix for the issue. We might support firefox better in the future.

Duplicate of #87