Slack – alt+click doesn’t mark a message as unread on slack webapp

From what I can see, you cannot post messages on Slack from the mobile web site. Even if you request the desktop version of the site there does not appear a way to post messages.

You'll need to install the Slack app.

This kind of feature is not supported by Slack.

I got in touch with them, and this is the official response:

Unfortunately, the option to hide on screen messages is not something we currently offer with Slack, although I see how useful this feature could be. I'm happy to pass this along as a suggestion to the team for future consideration! My apologies we don't currently offer this feature or something similar. (Amie, Slack Support)