compose: Fix "You need to scroll" notifications for large messages.

When you just sent a large message, our logic for "you need to scroll"
notifications did not correctly take into account the height of the
compose box.  This was easily reproduced when sending very long
messages.  The correct solution requires a bit of math to compute what
the visible area will look like after the compose box is closed.

This should be the final fix to #11138.

static/js/compose_actions.js

@@ -78,6 +78,7 @@ function show_box(msg_type, opts) {
     }
     $("#compose-send-status").removeClass(common.status_classes).hide();
     $('#compose').css({visibility: "visible"});
+    // When changing this, edit the 42px in _maybe_autoscroll
     $(".new_message_textarea").css("min-height", "3em");
 
     exports.set_focus(msg_type, opts);