Add more test cases

lib/mod_list_troubleshooter.dart

@@ -49,27 +49,54 @@ class ModListTroubleshooter {
   }
 
   ModList linearForward({int stepSize = 20}) {
-    _endIndex = _startIndex + stepSize;
+    final totalMods = originalModList.activeMods.length;
+    
+    if (_startIndex >= totalMods || _startIndex == _endIndex) {
+      int newStart = totalMods - stepSize;
+      if (newStart < 0) {
+        newStart = 0;
+      }
+      _startIndex = newStart;
+    }
+
+    _startIndex = _startIndex.clamp(0, totalMods);
+    _endIndex = (_startIndex + stepSize).clamp(0, totalMods);
+
     final subset = originalModList.activeMods.keys.toList().sublist(
       _startIndex,
       _endIndex,
     );
+
     currentModList.disableAll();
     currentModList.enableMods(subset);
     _startIndex = _endIndex;
+    
     return currentModList;
   }
 
   ModList linearBackward({int stepSize = 20}) {
-    _startIndex = _endIndex - stepSize;
-    _endIndex = _startIndex;
+    final totalMods = originalModList.activeMods.length;
+
+    if (_endIndex <= 0 || _startIndex == _endIndex) {
+      int newEnd = stepSize;
+      if (newEnd > totalMods) {
+        newEnd = totalMods;
+      }
+      _endIndex = newEnd;
+    }
+
+    _endIndex = _endIndex.clamp(0, totalMods);
+    _startIndex = (_endIndex - stepSize).clamp(0, _endIndex);
+
     final subset = originalModList.activeMods.keys.toList().sublist(
       _startIndex,
       _endIndex,
     );
+
     currentModList.disableAll();
     currentModList.enableMods(subset);
     _endIndex = _startIndex;
+    
     return currentModList;
   }