Excessive line spacing

In a new user I'm getting large line spacing when writing new messages:

https://i.imgur.com/zaSTk2z.png

That large signature should be single spaced. Even the cursor is double its normal height.

What I've checked:

• The code generated is fine, simple paragraphs or line breaks if we use ctrl-enter - for the recipients of the messages everything is fine;
• No userChrome;
• Checked the windows display configuration and it seems to be OK;

What else can this be?