fix: store the window's own filter in Session Files, not the global list

CONTEXT.md

@@ -130,6 +130,15 @@ split is two *apply phases* of the **Session Snapshot**, not a partial read).
   results/history → trim history). Used inside the Serial engine, per line by
   the Log Window's tail filter path, and per hit by Filter Pipes — all three
   adopt existing lists, so a Filter Run's history is continued in place.
+- **Window Filter** (`LogWindow._filterParams`) — The one filter a Log Window
+  currently holds in its filter panel. Per-window state: it is what a **Session
+  File** saves and restores, and it is not an entry of the **Saved Filter
+  List**. A Filter Pipe tab gets its own, cloned when the tab is created.
+- **Saved Filter List** (`Settings.FilterList`) — The application-wide list of
+  filters the user explicitly saved from the filter panel, shown in the filter
+  save list. Global, never per-window, and never restored from a Session File.
+  (Session Files written before the Window Filter change stored a copy of it
+  instead, of which only the first entry was ever restored.)
 - **Filter Spread** — Context expansion around a filter hit: **Back Spread**
   (`FilterParams.SpreadBefore`) lines before and **Fore Spread**
   (`FilterParams.SpreadBehind`) lines after the hit are included in the
@@ -146,7 +155,8 @@ split is two *apply phases* of the **Session Snapshot**, not a partial read).
 Spread**), "additional filter results" (the old internal name — use
 **Filter Spread**), "multi-threaded filter" as a concept name (it is a
 preference selecting the Parallel **Filter Engine**), "FilterFx" (legacy
-delegate name, deleted).
+delegate name, deleted), "the filter list" when the **Window Filter** is meant
+(the list is the **Saved Filter List**).
 
 ## Log Search
 

src/LogExpert.Core/Classes/Persister/PersistenceData.cs

@@ -38,6 +38,11 @@ public class PersistenceData
 
     public bool FilterAdvanced { get; set; }
 
+    /// <summary>
+    /// The Log Window's own filter, as the single entry of a list. A list for backward
+    /// compatibility only — <see cref="SessionFileComposer"/> owns the mapping and documents what
+    /// older Session Files carry here.
+    /// </summary>
     public List<FilterParams> FilterParamsList { get; set; } = [];
 
     public int FilterPosition { get; set; } = 222;

src/LogExpert.Core/Classes/Persister/SessionFileComposer.cs

@@ -13,6 +13,10 @@ public static class SessionFileComposer
     /// (the dead fields and <c>SessionFileName</c> named in the spec) are left at their type
     /// defaults. Note: <see cref="Persister"/> rewrites <c>FileName</c> on SameDir saves *after*
     /// compose — transformed by design, outside this contract.
+    /// <para>
+    /// <c>FilterParamsList</c> is a list only for backward compatibility: the window's own filter
+    /// goes in as its single entry, and a snapshot without one composes to an empty list.
+    /// </para>
     /// </summary>
     public static PersistenceData Compose (SessionSnapshot snapshot)
     {
@@ -39,7 +43,7 @@ public static PersistenceData Compose (SessionSnapshot snapshot)
             BookmarkList = snapshot.BookmarkList,
             RowHeightList = snapshot.RowHeightList,
             MultiFileNames = snapshot.MultiFileNames,
-            FilterParamsList = snapshot.FilterParamsList,
+            FilterParamsList = snapshot.FilterParams is null ? [] : [snapshot.FilterParams],
             Columnizer = snapshot.Columnizer,
             FilterTabDataList =
             [
@@ -55,6 +59,11 @@ public static PersistenceData Compose (SessionSnapshot snapshot)
     /// <summary>
     /// Maps a loaded Session File back to a snapshot. Fields the snapshot does not carry are
     /// ignored.
+    /// <para>
+    /// The window's filter is <c>FilterParamsList[0]</c>, which is what load has always used.
+    /// Session Files written before the per-window filter change carry a copy of the whole global
+    /// filter list there; the trailing entries were never restored by anything and are dropped.
+    /// </para>
     /// </summary>
     public static SessionSnapshot Decompose (PersistenceData persistenceData)
     {
@@ -81,7 +90,7 @@ public static SessionSnapshot Decompose (PersistenceData persistenceData)
             BookmarkList = persistenceData.BookmarkList,
             RowHeightList = persistenceData.RowHeightList,
             MultiFileNames = persistenceData.MultiFileNames,
-            FilterParamsList = persistenceData.FilterParamsList,
+            FilterParams = persistenceData.FilterParamsList.Count > 0 ? persistenceData.FilterParamsList[0] : null,
             Columnizer = persistenceData.Columnizer,
             FilterTabs =
             [

src/LogExpert.Core/Classes/Persister/SessionSnapshot.cs

@@ -67,7 +67,12 @@ public class SessionSnapshot
 
     public List<string> MultiFileNames { get; set; } = [];
 
-    public List<FilterParams> FilterParamsList { get; set; } = [];
+    /// <summary>
+    /// The window's own filter, or <c>null</c> when there is none to save (Save Filters off, or a
+    /// Session File that carries no filter). Its serialized form is a one-entry list — see
+    /// <see cref="SessionFileComposer"/>.
+    /// </summary>
+    public FilterParams FilterParams { get; set; }
 
     public ILogLineMemoryColumnizer Columnizer { get; set; }
 

src/LogExpert.Persister.Tests/SessionFileComposerTests.cs

@@ -177,7 +177,7 @@ private static SessionSnapshot CreateFullSnapshot (int filterTabDepth = 2)
             BookmarkList = new SortedList<int, Bookmark> { [5] = new Bookmark(5, "a manual bookmark") },
             RowHeightList = new SortedList<int, RowHeightEntry> { [3] = new RowHeightEntry(3, 60) },
             MultiFileNames = ["app.1.log", "app.2.log"],
-            FilterParamsList = [CreateFilterParams("ERROR")],
+            FilterParams = CreateFilterParams("ERROR"),
             Columnizer = new DefaultLogfileColumnizer(),
             FilterTabs = filterTabDepth > 0
                 ?
@@ -369,6 +369,66 @@ public void DiskTrip_ComposePersistLoadDecompose_ReturnsEqualSnapshot ()
 
     #endregion
 
+    #region Window filter (issue #666)
+
+    // A Session File carries the window's OWN filter, not a copy of the global filter list. The
+    // on-disk shape stays a list so existing .lxp files keep loading — new saves just write
+    // exactly one entry into it.
+    [Test]
+    public void Compose_WritesTheWindowFilterAsTheSingleFilterParamsListEntry ()
+    {
+        var snapshot = CreateFullSnapshot(filterTabDepth: 0);
+        snapshot.FilterParams = CreateFilterParams("the window's own filter");
+
+        var composed = SessionFileComposer.Compose(snapshot);
+
+        Assert.That(composed.FilterParamsList, Has.Count.EqualTo(1));
+        Assert.That(composed.FilterParamsList[0].SearchText, Is.EqualTo("the window's own filter"));
+    }
+
+    // SaveFilters off (and filter-less windows) leave the snapshot without a filter — the field
+    // must not serialize a null element into the list.
+    [Test]
+    public void Compose_WithoutAWindowFilter_WritesAnEmptyFilterParamsList ()
+    {
+        var snapshot = CreateFullSnapshot(filterTabDepth: 0);
+        snapshot.FilterParams = null;
+
+        Assert.That(SessionFileComposer.Compose(snapshot).FilterParamsList, Is.Empty);
+    }
+
+    // The compatibility story: a pre-#666 Session File carries a copy of the whole global filter
+    // list, and load has always taken [0] as the window's filter. That stays true — and the
+    // trailing entries, which nothing ever restored, are dropped when such a file is re-saved.
+    [Test]
+    public void Decompose_LegacySessionFileCarryingTheGlobalList_KeepsTheFirstEntryAndDropsTheRest ()
+    {
+        var legacy = CreateCarriedPersistenceData(filterTabDepth: 0);
+        legacy.FilterParamsList =
+        [
+            CreateFilterParams("the first global filter"),
+            CreateFilterParams("another global filter"),
+        ];
+
+        var snapshot = SessionFileComposer.Decompose(legacy);
+        var recomposed = SessionFileComposer.Compose(snapshot);
+
+        Assert.That(snapshot.FilterParams.SearchText, Is.EqualTo("the first global filter"));
+        Assert.That(recomposed.FilterParamsList, Has.Count.EqualTo(1));
+        Assert.That(recomposed.FilterParamsList[0].SearchText, Is.EqualTo("the first global filter"));
+    }
+
+    [Test]
+    public void Decompose_SessionFileWithoutFilters_LeavesTheWindowFilterUnset ()
+    {
+        var data = CreateCarriedPersistenceData(filterTabDepth: 0);
+        data.FilterParamsList = [];
+
+        Assert.That(SessionFileComposer.Decompose(data).FilterParams, Is.Null);
+    }
+
+    #endregion
+
     #region Omitted fields (the ✖ rows of the mapping table)
 
     // The snapshot does not carry the dead fields or SessionFileName, so Compose must leave them

src/LogExpert.UI/Controls/LogWindow/LogWindow.cs

@@ -2660,9 +2660,9 @@ private void LoadPersistenceData ()
     [SupportedOSPlatform("windows")]
     private void RestoreFilters (SessionSnapshot snapshot)
     {
-        if (snapshot.FilterParamsList.Count > 0)
+        if (snapshot.FilterParams != null)
         {
-            _filterParams = snapshot.FilterParamsList[0];
+            _filterParams = snapshot.FilterParams;
             ReInitFilterParams(_filterParams);
         }
 
@@ -5832,8 +5832,8 @@ public string SavePersistenceDataAndReturnFileName (bool force)
         try
         {
             // Relocated from the old composing getter (the composer must stay side-effect-free):
-            // this option doesn't need a press on 'search'. The write must precede the gather —
-            // the gather copies the global FilterList, and this write may alias into it.
+            // this option doesn't need a press on 'search'. The write must precede the gather,
+            // which is what copies _filterParams into the snapshot.
             _filterParams.IsFilterTail = filterTailCheckBox.Checked;
 
             var persistenceData = SessionFileComposer.Compose(GatherSessionSnapshot());
@@ -5893,8 +5893,12 @@ public SessionSnapshot GatherSessionSnapshot ()
 
         if (Preferences.SaveFilters)
         {
-			// Persist this window's active filter; FilterList contains global saved presets.
-			snapshot.FilterParamsList = [_filterParams.Clone()];
+            // The window's own filter, not the global filter list: a Session File is per-window
+            // state, and the global list is never restored from one anyway. Clone() is a shallow
+            // copy (ColumnList and the columnizer stay shared) — enough here, because compose and
+            // serialize run synchronously right after this gather.
+            snapshot.FilterParams = _filterParams.Clone();
+
             foreach (var filterPipe in _filterPipeList)
             {
                 snapshot.FilterTabs.Add(new FilterTabSnapshot