[gtk] only new messages mark as animatable

gtk/src/transcript/transcript-drawing-area.vala

@@ -383,6 +383,10 @@ private class TranscriptDrawingArea : Widget
                 items.add(image_layout);
             }
 
+            // New-message animation is a one-shot effect. Clear the flag after
+            // scheduling bubble animations so later relayouts do not replay it.
+            message.should_animate = false;
+
             last_sender = message.sender;
             last_date = date;