Editing messages that contain markdown quotes inserts a newline after the `>`

[jeffcasavant]

Description

When editing a message containing a markdown quote, Riot will insert a newline that has to be manually removed to retain the quote formatting.

> This is my original message

when edited, becomes

>
This is my original message.

Steps to reproduce

• Send a message of the format above.
• Begin to edit the message either via the pencil button or the up arrow.

Logs being sent: yes

Version information

• Riot-Desktop 1.5.10
• macOS

[AgentOak]

This is super annoying because of #16069. About half of the time when I quote something I forget to do a double newline, like so

>quote
comment

Element includes both lines in the quote so I edit my message to insert the extra newline, but then Element shows the message source as this:

> 
quote
> comment

Not only has it inserted a newline like OP describes, it also inserted multiple unnecessary spaces and an additional quote character. This is quite a mess and super annoying because it requires multiple edits/jumps to undo all the changes.

This issue probably shares a root cause with #13474.

(Confirm for Element 1.9.0)