HUD/PrearmStatus: fix failure messages

When prearm checks are failing, the HUD displays the last failure message,
even if that message is very stale. This changes it so it displays the
latest failure message since the last time it passed the prearm check. The
prearm failure messages were also moved to the lowest priority, so that they
would not mask other sensor failures.

The PrearmStatus window had a similar problem, where it would show all
failure messages briefly when it first launched, instead of the latest
relevant ones.

Also, a drive-by fix of where the messageHigh setter function sets the
_messageHighTime variable, which was causing flickers.

Controls/PrearmStatus.cs

@@ -16,8 +16,8 @@ public partial class PrearmStatus : Form
     {
         private static readonly ILog log = LogManager.GetLogger(MethodBase.GetCurrentMethod().DeclaringType);
 
-        private DateTime lastRequestTime = DateTime.MinValue;
-        private DateTime searchTime = DateTime.MinValue;
+        private DateTime lastRequestTime = DateTime.MaxValue;
+        private DateTime searchTime = DateTime.MaxValue;
         public PrearmStatus()
         {
             InitializeComponent();

ExtLibs/ArduPilot/CurrentState.cs

@@ -174,6 +174,8 @@ public bool prearmstatus
             get => connected && (sensors_health.prearm || !sensors_enabled.prearm);
         }
 
+        private DateTime last_good_prearm = DateTime.MaxValue;
+
         private bool useLocation;
 
         /// <summary>
@@ -1105,10 +1107,10 @@ public string messageHigh
                 if (value == null || value == "")
                     return;
                 // check against get
+                _messageHighTime = DateTime.Now;
                 if (messageHigh == value)
                     return;
                 log.Info("messageHigh " + value);
-                _messageHighTime = DateTime.Now;
                 _messagehigh = value;
                 messageHighSeverity = MAVLink.MAV_SEVERITY.EMERGENCY;
             }
@@ -2687,12 +2689,7 @@ private void Parent_OnPacketReceived(object sender, MAVLink.MAVLinkMessage mavLi
                                 messageHigh = "InternalError 0x" + (errors_count1 + (errors_count2 << 16)).ToString("X");
                             }
 
-                            if (!sensors_health.prearm && sensors_enabled.prearm && sensors_present.prearm)
-                            {
-                                messageHigh = messages.LastOrDefault(a => a.message.ToLower().Contains("prearm")).message
-                                    ?.ToString();
-                            }
-                            else if (!sensors_health.gps && sensors_enabled.gps && sensors_present.gps)
+                            if (!sensors_health.gps && sensors_enabled.gps && sensors_present.gps)
                             {
                                 messageHigh = Strings.BadGPSHealth;
                             }
@@ -2768,6 +2765,16 @@ private void Parent_OnPacketReceived(object sender, MAVLink.MAVLinkMessage mavLi
                             {
                                 messageHigh = Strings.BadAirspeed;
                             }
+                            else if (!sensors_health.prearm && sensors_enabled.prearm && sensors_present.prearm)
+                            {
+                                messageHigh = messages.LastOrDefault(a =>
+                                        a.message.ToLower().Contains("prearm") &&
+                                        a.time > last_good_prearm).message?.ToString();
+                            }
+                            if (last_good_prearm > DateTime.Now || sensors_health.prearm || !sensors_enabled.prearm || !sensors_present.prearm)
+                            {
+                                last_good_prearm = DateTime.Now;
+                            }
                         }
 
                         break;