fixed lesson merging mutation, improved overlap detection, and implemented priority-based lane assignment with tablet support

lib/view/pages/timetable/data/timetable_appointment_factory.dart

@@ -148,7 +148,13 @@ class TimetableAppointmentFactory {
     return cleaned.isEmpty ? null : cleaned;
   }
 
-  // Pure: returns a new list, does not mutate input.
+  // Pure: returns a new list of fresh objects, does not mutate input.
+  // (The previous version replaced `previous.endTime` in place, which
+  // mutated the original lesson object passed in via [input]. Across
+  // rebuilds those mutated lessons were observed again by the next merge
+  // pass — extending lessons further or, after the overlap-gap guard was
+  // added to [_canMerge], even causing the second half of a double lesson
+  // to be emitted alongside the already-merged block.)
   static List<GetTimetableResponseObject> _mergeAdjacentLessons(
     List<GetTimetableResponseObject> input, {
     Duration maxGap = const Duration(minutes: 5),
@@ -158,19 +164,22 @@ class TimetableAppointmentFactory {
     final sorted = [...input]..sort((a, b) =>
         WebuntisTime.parse(a.date, a.startTime).compareTo(WebuntisTime.parse(b.date, b.startTime)));
 
-    final merged = <GetTimetableResponseObject>[sorted.first];
-    for (var i = 1; i < sorted.length; i++) {
-      final previous = merged.last;
-      final current = sorted[i];
-      if (_canMerge(previous, current, maxGap)) {
-        previous.endTime = current.endTime;
+    final merged = <GetTimetableResponseObject>[];
+    for (final current in sorted) {
+      if (merged.isNotEmpty && _canMerge(merged.last, current, maxGap)) {
+        // `merged.last` is always a copy we created below, so mutating its
+        // endTime is safe and keeps the next iteration's gap check correct.
+        merged.last.endTime = current.endTime;
       } else {
-        merged.add(current);
+        merged.add(_copyLesson(current));
       }
     }
     return merged;
   }
 
+  static GetTimetableResponseObject _copyLesson(GetTimetableResponseObject l) =>
+      GetTimetableResponseObject.fromJson(l.toJson());
+
   static bool _canMerge(GetTimetableResponseObject a, GetTimetableResponseObject b, Duration maxGap) {
     final aSubject = a.su.firstOrNull?.id;
     final bSubject = b.su.firstOrNull?.id;
@@ -179,7 +188,12 @@ class TimetableAppointmentFactory {
     if (a.te.firstOrNull?.id != b.te.firstOrNull?.id) return false;
     if (a.code != b.code) return false;
 
+    // Merge only sequential lessons (b starts at or after a ends, within the
+    // tolerance). Without the lower bound, identical-metadata lessons that
+    // overlap in time would silently collapse into one — and because the
+    // merge sets `previous.endTime = current.endTime`, an overlapping merge
+    // can even truncate the earlier lesson.
     final gap = WebuntisTime.parse(b.date, b.startTime).difference(WebuntisTime.parse(a.date, a.endTime));
-    return gap <= maxGap;
+    return !gap.isNegative && gap <= maxGap;
   }
 }