Comment 33 for bug 1771880

Off-topic for Seahorse, but the reporter's (and my) real problem is about signing in LibreOffice.
I've found a way to sign document in LibreOffice on Ubuntu:

LibreOffice searches a keystore in this order:
a.) The environment variable MOZILLA_CERTIFICATE_FOLDER
b.) The Thunderbird profile
c.) The Mozilla suite profile
d.) The Firefox profile.
(source: https://wiki.openoffice.org/wiki/How_to_use_digital_Signatures )

So you do not need seahorse to sign documents in LibreOffice, only Thunderbird or Firefox.

Steps:
1. Import your certificate into Thunderbird or Firefox key store (Edit, Preferences, etc.).
2. LO help says: "It is also necessary that the trust settings for the root certificates are set to trust the certificate to identify web sites and e-mail users."
Make sure they are set.
3. In LibreOffice, Tools, Options, LibreOffie\Security, in "Certificate Path" the first keystore (for me) is "/home/lml/.thunderbird/something.default".
(Note:
  - It seems you can choose between thunderbird and firefox keystore, but you can't.
  - The LO help mentions the "Certificate Detection" page,
    the Basic script there gives the same result.
)
Override this detected value using the following command to start LibreOffice using terminal:

MOZILLA_CERTIFICATE_FOLDER=sql:/home/lml/.thunderbird/something.default soffice

(use your own path for Thunderbird or Firefox keystore)

The trick is to add "sql:" to the beginning of the value and override the (otherwise correct) detected path. Maybe there is a problem in LibreOffice not able to use the certificate folder...

I'm using Ubuntu 19.10, LibreOffice 6.3.2, Thunderbird 60.9.