offline: Reduce plymouth messages flickering

The fact that plymouth prints messages centered causes unpleasant
flickering during the offline transaction execution.
Reduce flickering by normalizing all the messages to the same size, so
the '[count/total]' part of the messages stays on the fixed position.

dnf5/commands/offline/offline.cpp

@@ -55,6 +55,7 @@ const sdbus::ObjectPath SYSTEMD_OBJECT_PATH{"/org/freedesktop/systemd1"};
 const SDBUS_INTERFACE_NAME_TYPE SYSTEMD_MANAGER_INTERFACE{"org.freedesktop.systemd1.Manager"};
 const SDBUS_INTERFACE_NAME_TYPE SYSTEMD_UNIT_INTERFACE{"org.freedesktop.systemd1.Unit"};
 const std::string SYSTEMD_SERVICE_NAME{"dnf5-offline-transaction.service"};
+const std::size_t PLYMOUTH_LINE_LENGTH{75};
 
 int call(const std::string & command, const std::vector<std::string> & args) {
     std::vector<char *> c_args;
@@ -99,11 +100,12 @@ class PlymouthOutput {
     bool ping() { return plymouth({"--ping"}); }
     bool set_mode() { return plymouth({"change-mode", "--system-upgrade"}); }
     bool message(const std::string & message) {
-        if (last_message.has_value() && message == last_message) {
+        auto msg = normalize_message(message);
+        if (last_message.has_value() && msg == last_message) {
             plymouth({"hide-message", "--text", last_message.value()});
         }
-        last_message = message;
-        return plymouth({"display-message", "--text", message});
+        last_message = msg;
+        return plymouth({"display-message", "--text", msg});
     }
     bool progress(const int percentage) {
         return plymouth({"system-update", "--progress", std::to_string(percentage)});
@@ -124,6 +126,14 @@ class PlymouthOutput {
         }
         return alive;
     }
+    std::string normalize_message(const std::string & original_message) {
+        std::string message{original_message};
+        const auto message_length = message.length();
+        if (message_length < PLYMOUTH_LINE_LENGTH) {
+            message.append(PLYMOUTH_LINE_LENGTH - message_length, ' ');
+        }
+        return message.substr(0, PLYMOUTH_LINE_LENGTH);
+    }
 };
 
 /// Extend RpmTransCB to also display messages with Plymouth