After enabling WP-Discourse plugin, I found comments from Discourse caused my WP site mix-content problem: All the avatars were loaded through HTTP instead of HTTPS.
I tried to fix it myself. And after reading the WP-discourse plugin code, I found the HTTP image URLs are stored in wp_postmeta with meta_key
discourse_comments_raw. And this value is got from https://my.discourse.site/t/topic/20/wordpress.json
I found this JSON file responded with HTTP images URLs instead of HTTPS. But my whole Discourse site is transferred through HTTPS.(Though I noticed that it also sometimes get mix-content problem too)
The possible reason for this is that I used HTTP from the very beginning and later changed to HTTPS and some settings seems not changed…
Can anybody tell me how to fix this?