fix: terminal input reaches the shell in the order it was typed

internal/server/handlers_panel_term.go

@@ -49,6 +49,9 @@ type panelInMsg struct {
 	D    string `json:"d,omitempty"`
 	Cols uint16 `json:"cols,omitempty"`
 	Rows uint16 `json:"rows,omitempty"`
+	// Seq numbers the messages of a session from 1; 0 from a page that
+	// predates it.
+	Seq uint64 `json:"seq,omitempty"`
 }
 
 // POST /api/panel/term/{sid}/in - send keystrokes/resize to the agent PTY.
@@ -63,6 +66,10 @@ func (s *Server) handlePanelIn(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "bad request", http.StatusBadRequest)
 		return
 	}
+	if m.Seq > 0 && !sess.TakeSeq(m.Seq) {
+		w.WriteHeader(http.StatusNoContent) // a repeat of one already taken
+		return
+	}
 	switch m.T {
 	case "i":
 		sess.PushIn(proto.InFrame{Type: proto.FrameStdin, Data: m.D})