Fixed issue JAM-Software#487: When OnAddToSelection is called, GetFir…

…stSelected returns nil

# Conflicts:
#	Source/VirtualTrees.pas

Source/VirtualTrees.pas

@@ -24264,8 +24264,9 @@ function TBaseVirtualTree.InternalAddToSelection(const NewItems: TNodeArray; New
   CurrentEnd: Integer;
   Constrained,
   SiblingConstrained: Boolean;
-
+  lPreviousSelectedCount: Integer;
 begin
+  lPreviousSelectedCount := FSelectionCount;
   // The idea behind this code is to use a kind of reverse merge sort. QuickSort is quite fast
   // and would do the job here too but has a serious problem with already sorted lists like FSelection.
 
@@ -24275,6 +24276,7 @@ function TBaseVirtualTree.InternalAddToSelection(const NewItems: TNodeArray; New
     for I := 0 to NewLength - 1 do
     begin
       Include(NewItems[I].States, vsSelected);
+      Inc(FSelectionCount);
       if Assigned(FOnAddToSelection) then
         FOnAddToSelection(Self, NewItems[I]);
     end;
@@ -24296,7 +24298,8 @@ function TBaseVirtualTree.InternalAddToSelection(const NewItems: TNodeArray; New
       else
       begin
         Include(NewItems[I].States, vsSelected);
-        if Assigned(FOnAddToSelection) then
+      Inc(FSelectionCount);
+      if Assigned(FOnAddToSelection) then
           FOnAddToSelection(Self, NewItems[I]);
       end;
   end;
@@ -24312,12 +24315,12 @@ function TBaseVirtualTree.InternalAddToSelection(const NewItems: TNodeArray; New
     if NewLength > 1 then
       QuickSort(NewItems, 0, NewLength - 1);
     // 3) Make room in FSelection for the new items.
-    if FSelectionCount + NewLength >= Length(FSelection) then
-      SetLength(FSelection, FSelectionCount + NewLength);
+    if lPreviousSelectedCount + NewLength >= Length(FSelection) then
+      SetLength(FSelection, lPreviousSelectedCount + NewLength);
 
     // 4) Merge in new items
     J := NewLength - 1;
-    CurrentEnd := FSelectionCount - 1;
+    CurrentEnd := lPreviousSelectedCount - 1;
 
     while J >= 0 do
     begin
@@ -24353,7 +24356,7 @@ function TBaseVirtualTree.InternalAddToSelection(const NewItems: TNodeArray; New
       CurrentEnd := I;
     end;
 
-    Inc(FSelectionCount, NewLength);
+    Assert(FSelectionCount = (lPreviousSelectedCount + NewLength), 'Fixing issue #487 seems to ahve caused a problem here.')
   end;
 end;