Fix change frame rate/speed corrupting selected line duration (#11650)

src/ui/Features/Main/SubtitleLineViewModel.cs

@@ -595,6 +595,24 @@ internal void SetStartTimeOnly(TimeSpan timeSpan)
         UpdateDuration();
     }
 
+    /// <summary>
+    /// Sets start and end time together without ever publishing an invalid
+    /// intermediate state (e.g. start &gt; end / negative duration). Updating
+    /// the two times separately can briefly expose such a state to the live
+    /// editor controls bound to the selected line; the duration up/down clamps
+    /// the negative value and writes it back, corrupting the end time. Suppress
+    /// notifications while both values are assigned, then recompute duration once.
+    /// </summary>
+    internal void SetTimes(TimeSpan startTime, TimeSpan endTime)
+    {
+        _skipUpdate = true;
+        StartTime = startTime;
+        EndTime = endTime;
+        _skipUpdate = false;
+
+        UpdateDuration();
+    }
+
     internal void Adjust(double factor, double adjustmentInSeconds)
     {
         if (StartTime.IsMaxTime())

src/ui/Features/Sync/ChangeFrameRate/ChangeFrameRateViewModel.cs

@@ -157,8 +157,9 @@ internal static void ChangeFrameRate(ObservableCollection<SubtitleLineViewModel>
         double ratio = fromFrameRate / toFrameRate;
         foreach (var line in subtitles)
         {
-            line.SetStartTimeOnly(TimeSpan.FromMilliseconds(line.StartTime.TotalMilliseconds * ratio));
-            line.EndTime = TimeSpan.FromMilliseconds(line.EndTime.TotalMilliseconds * ratio);
+            line.SetTimes(
+                TimeSpan.FromMilliseconds(line.StartTime.TotalMilliseconds * ratio),
+                TimeSpan.FromMilliseconds(line.EndTime.TotalMilliseconds * ratio));
         }
     }
 }

src/ui/Features/Sync/ChangeSpeed/ChangeSpeedViewModel.cs

@@ -86,8 +86,10 @@ internal static void ChangeSpeed(ObservableCollection<SubtitleLineViewModel> sub
     {
         foreach (var subtitle in subtitles)
         {
-            subtitle.SetStartTimeOnly(TimeSpan.FromMilliseconds(subtitle.StartTime.TotalMilliseconds * (100.0 / speedPercent)));
-            subtitle.EndTime = TimeSpan.FromMilliseconds(subtitle.EndTime.TotalMilliseconds * (100.0 / speedPercent));
+            var factor = 100.0 / speedPercent;
+            subtitle.SetTimes(
+                TimeSpan.FromMilliseconds(subtitle.StartTime.TotalMilliseconds * factor),
+                TimeSpan.FromMilliseconds(subtitle.EndTime.TotalMilliseconds * factor));
         }
     }
 }