I was using Slack and even though I was clearly connected, the messages from my peer were greyed out. Switching chats and then switching back fixed it, so hard to tell if it's Stylo or not, but suggestive.
We have some bugs about the formatting of Slack channel names changing intermittently (bug 1378987 and bug 1380102), so Stylo might be tripping on some common styling code that Slack uses.
:ekr, does this still persist? I think either bug 1371450 and bug 1381431 fixed this issue.