The context menu item `Clear the selected message and previous messages'
Let `subwindow X' be a subwindow with chat messages for a person-to-person chat or for a conference chat.
All that our parties unexpectedly tell in subwindow X could be blindly cleared with the current Ctrl L / `Clear' feature.
The user needs to explicitly select the last message she/he has in fact read, without a faulty expectation that she/he has read all of the subwindow X, since the subwindow X might have highly dynamic (very volatile) nature.
I think, the right click menu on a chat message (the context menu of a chat message) in the subwindow X should select a chat message (with some kind of visual/audible indication, and should contain a menu item like `Clear the selected message and previous messages: I've read these messages' which:
- does wipe (clear) the selected message and prevous messages from the screen
- does NOT touch (doesn't clear) the messages which are newer than the selected message.
This way, we
- clear the chat from everything we've already read, and
- we do not miss newest messages if these are unexpectedly added by our chat parties, so we are safer and more calm about hearing all that our parties unexpectedly tell in the subwindow X.
 ...from the screen, and*or from the other subwindow X's display devices.