LCL: first try at implementing TMenuItem.AutoLineReduction. Based upon patch by Artem Izmaylov, modified by me.

lcl/include/menu.inc

@@ -22,6 +22,7 @@
 constructor TMenu.Create(AOwner: TComponent);
 begin
   FItems := TMenuItem.Create(Self);
+  FItems.FAutoLineReduction := maAutomatic;
   FItems.FOnChange := @MenuChanged;
   FItems.FMenu := Self;
   FImageChangeLink := TChangeLink.Create;
@@ -93,6 +94,14 @@ begin
   FItems.UpdateImages;
 end;
 
+function TMenu.GetAutoLineReduction: TMenuAutoFlag;
+begin
+  case FItems.AutoLineReduction of
+    maParent, maAutomatic: Result := maAutomatic;  //cannot return maParent for TMenu.AutoLineReduction
+    maManual: Result := maManual;
+  end;
+end;
+
 procedure TMenu.BidiModeChanged;
 begin
   if HandleAllocated then
@@ -140,6 +149,11 @@ begin
   if Sender = Images then UpdateItems;
 end;
 
+procedure TMenu.SetAutoLineReduction(AValue: TMenuAutoFlag);
+begin
+  FItems.AutoLineReduction := AValue;
+end;
+
 {------------------------------------------------------------------------------
   Method:  TMenu.CreateHandle
   Params:  None

lcl/include/menuitem.inc

@@ -115,6 +115,7 @@ begin
   FRightJustify := False;
   FShowAlwaysCheckable := False;
   FGlyphShowMode := gsmApplication;
+  FAutoLineReduction := maParent;
 
   FImageChangeLink := TChangeLink.Create;
   FImageChangeLink.OnChange := @ImageListChange;
@@ -342,6 +343,69 @@ begin
   end;
 end;
 
+function TMenuItem.InternalRethinkLines(AForced: Boolean): Boolean;
+var
+  LItem: TMenuItem;
+  LLastVisibleIsLine, HideSeps: Boolean;
+  LLastVisibleItem: TMenuItem;
+  I: Integer;
+begin
+  debugln(['TMenuItem.InternalRethinkLines, AForced=',AForced]);
+  Result := False;
+  if (csDesigning in ComponentState) then
+    Exit;
+  HideSeps := GetAutoLineReduction;
+  if {AForced or} HideSeps then
+  begin
+    LLastVisibleIsLine := True; // to hide head line
+    LLastVisibleItem := nil;
+
+    for I:= 0 to Count - 1 do
+    begin
+      LItem := Items[I];
+      if LItem.IsLine then
+      begin
+        // hide head or double lines
+        if LItem.Visible <> not LLastVisibleIsLine then
+        begin
+          LItem.Visible := not LLastVisibleIsLine;
+          Result := True;
+          debugln(['TMenuItem.InternalRethinkLines: hiding separator with name "',LItem.Name,'"']);
+        end;
+      end;
+
+      if LItem.Visible then
+      begin
+        LLastVisibleIsLine := LItem.IsLine;
+        LLastVisibleItem := LItem;
+      end;
+    end;
+
+    // hide end line
+    if (LLastVisibleItem <> nil) and
+      LLastVisibleItem.IsLine and LLastVisibleItem.Visible then
+    begin
+      LLastVisibleItem.Visible := False;
+      Result := True;
+      debugln(['TMenuItem.InternalRethinkLines: hiding separator with name "',LItem.Name,'"']);
+    end;
+  end
+  else
+  //apparently if GetAToLineReduction is False, Delphi sets all separators to be visible
+  begin
+    for i := 0 to Count - 1 do
+    begin
+      LItem := Items[i];
+      if LItem.IsLine and not LItem.Visible then
+      begin
+        LItem.Visible := True;
+        Result := True;
+        debugln(['TMenuItem.InternalRethinkLines: unhiding separator with name "',LItem.Name,'"']);
+      end;
+    end;
+  end;
+end;
+
 procedure TMenuItem.CheckChildrenHandles;
 
   function GetMenu(Item: TMenuItem): TMenu;
@@ -370,6 +434,14 @@ begin
   if FItems = nil then
     Exit;
 
+  InitiateActions; // actions may update items visibility
+  if InternalRethinkLines(False) then
+  begin
+    // Especially to re-index the items in native way
+    for i := 0 to Count - 1 do
+      Items[I].DestroyHandle;
+  end;
+
   AMenu := GetMenu(Self);
   AMergedItems := MergedItems;
   for i := 0 to AMergedItems.InvisibleCount-1 do
@@ -691,6 +763,15 @@ begin
     RecreateHandle;
 end;
 
+procedure TMenuItem.SetAutoLineReduction(AValue: TMenuItemAutoFlag);
+begin
+  if FAutoLineReduction <> AValue then
+  begin
+    FAutoLineReduction := AValue;
+    MenuChanged(True);
+  end;
+end;
+
 {------------------------------------------------------------------------------
   Function: TMenuItem.GetParentMenu
   Params:   none
@@ -1053,6 +1134,13 @@ begin
   end;
 end;
 
+function TMenuItem.RethinkLines: Boolean;
+begin
+  Result := InternalRethinkLines(True);
+  if Result then
+    MenuChanged(True);
+end;
+
 procedure TMenuItem.RemoveAllHandlersOfObject(AnObject: TObject);
 var
   HandlerType: TMenuItemHandlerType;
@@ -1673,4 +1761,20 @@ begin
   UpdateImage;
 end;
 
+function TMenuItem.GetAutoLineReduction: Boolean;
+var
+  AutoFlag: TMenuItemAutoFlag;
+begin
+  AutoFlag := FAutoLineReduction;
+  if (AutoFlag = maParent) and Assigned(Parent) then
+    if Parent.GetAutoLineReduction then
+      AutoFlag := maAutomatic
+    else
+      AutoFlag := maManual;
+  case AutoFlag of
+    maParent, maAutomatic: Result := True;
+    maManual: Result := False;
+  end;
+end;
+
 // included by menus.pp

lcl/menus.pp

@@ -59,6 +59,8 @@ type
 
   TMenuChangeEvent = procedure (Sender: TObject; Source: TMenuItem;
                                 Rebuild: Boolean) of object;
+  TMenuItemAutoFlag = (maAutomatic, maManual, maParent);
+  TMenuAutoFlag = maAutomatic..maManual;
 
   { TMenuActionLink }
 
@@ -147,6 +149,7 @@ type
   TMenuItem = class(TLCLComponent)
   private
     FActionLink: TMenuActionLink;
+    FAutoLineReduction: TMenuItemAutoFlag;
     FCaption: TTranslateString;
     FBitmap: TBitmap;
     FGlyphShowMode: TGlyphShowMode;
@@ -200,6 +203,7 @@ type
     function IsVisibleStored: boolean;
     procedure MergeWith(const aMenu: TMenuItem);
     procedure SetAutoCheck(const AValue: boolean);
+    procedure SetAutoLineReduction(AValue: TMenuItemAutoFlag);
     procedure SetCaption(const AValue: TTranslateString);
     procedure SetChecked(AValue: Boolean);
     procedure SetDefault(AValue: Boolean);
@@ -222,9 +226,11 @@ type
     class procedure WSRegisterClass; override;
     procedure ActionChange(Sender: TObject; CheckDefaults: Boolean); virtual;
     procedure AssignTo(Dest: TPersistent); override;
+    function GetAutoLineReduction: Boolean;
     procedure BitmapChange(Sender: TObject);
     function DoDrawItem(ACanvas: TCanvas; ARect: TRect; AState: TOwnerDrawState): Boolean; virtual;
     function DoMeasureItem(ACanvas: TCanvas; var AWidth, AHeight: Integer): Boolean; virtual;
+    function InternalRethinkLines(AForced: Boolean): Boolean; virtual;
     function GetAction: TBasicAction;
     function GetActionLinkClass: TMenuActionLinkClass; virtual;
     function GetHandle: HMenu;
@@ -287,6 +293,7 @@ type
     procedure Clear;
     function HasBitmap: boolean;
     function GetIconSize(ADC: HDC; DPI: Integer = 0): TPoint; virtual;
+    function RethinkLines: Boolean;
     // Event lists
     procedure RemoveAllHandlersOfObject(AnObject: TObject); override;
     procedure AddHandlerOnDestroy(const OnDestroyEvent: TNotifyEvent;
@@ -313,6 +320,8 @@ type
   published
     property Action: TBasicAction read GetAction write SetAction;
     property AutoCheck: boolean read FAutoCheck write SetAutoCheck default False;
+    property AutoLineReduction: TMenuItemAutoFlag
+      read FAutoLineReduction write SetAutoLineReduction default maParent;
     property Caption: TTranslateString read FCaption write SetCaption
                              stored IsCaptionStored;
     property Checked: Boolean read FChecked write SetChecked
@@ -367,8 +376,10 @@ type
 //See TCustomForm.CMBiDiModeChanged
     procedure CMParentBiDiModeChanged(var Message: TLMessage); message CM_PARENTBIDIMODECHANGED;
     procedure CMAppShowMenuGlyphChanged(var Message: TLMessage); message CM_APPSHOWMENUGLYPHCHANGED;
+    function GetAutoLineReduction: TMenuAutoFlag;
     function IsBiDiModeStored: Boolean;
     procedure ImageListChange(Sender: TObject);
+    procedure SetAutoLineReduction(AValue: TMenuAutoFlag);
     procedure SetBiDiMode(const AValue: TBiDiMode);
     procedure SetImages(const AValue: TCustomImageList);
     procedure SetImagesWidth(const aImagesWidth: Integer);
@@ -411,6 +422,8 @@ type
     property Parent: TComponent read FParent write SetParent;
     property ShortcutHandled: boolean read FShortcutHandled write FShortcutHandled;
   published
+    property AutoLineReduction: TMenuAutoFlag
+      read GetAutoLineReduction write SetAutoLineReduction default maAutomatic;
     property BidiMode:TBidiMode read FBidiMode write SetBidiMode stored IsBiDiModeStored default bdLeftToRight;
     property ParentBidiMode:Boolean read FParentBidiMode write SetParentBidiMode default True;
     property Items: TMenuItem read FItems;