How to keep label smoothly centered in scrollbox?

Question

I use a TMemo in TScrollBox to show some text, and a TLabel on top as a header info. Sometimes memo is wider than scroll box and of course Horizontal scroll bar can be used to scroll left and right to see text in memo. I want to have a label as a header always centered to scroll box visible area. I can do this by setting Label1.Left:= (Scrollbox1.Width div 2) - (Label1.Width div 2); and it works but it kind of flickers, shakes when scrolling back and forth. Memo moves smoothly, label doesn't.

Here is unit:

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type

  TScrollBox=Class(VCL.Forms.TScrollBox)
    procedure WMHScroll(var Message: TWMHScroll); message WM_HSCROLL;
  private
    FOnScrollHorz: TNotifyEvent;
  public
   Property OnScrollHorz:TNotifyEvent read FOnScrollHorz Write FonScrollHorz;
  End;

  TForm1 = class(TForm)
    ScrollBox1: TScrollBox;
    Label1: TLabel;
    Memo1: TMemo;
    procedure FormCreate(Sender: TObject);
    procedure ScrollBox1Resize(Sender: TObject);
  private
    procedure MyScrollHorz(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TScrollBox.WMHScroll(var Message: TWMHScroll);
begin
   inherited;
   if Assigned(FOnScrollHorz) then  FOnScrollHorz(Self);
end;

procedure TForm1.MyScrollHorz(Sender: TObject);
begin
    Label1.Left:= (Scrollbox1.Width div 2) - (Label1.Width div 2);
end;

procedure TForm1.ScrollBox1Resize(Sender: TObject);
begin
  Label1.Left:= (Scrollbox1.Width div 2) - (Label1.Width div 2);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  ScrollBox1.OnScrollHorz := MyScrollHorz;
end;

end.

and dfm:

object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 212
  ClientWidth = 458
  Color = clBtnFace
  DoubleBuffered = True
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  OnCreate = FormCreate
  PixelsPerInch = 96
  TextHeight = 13
  object ScrollBox1: TScrollBox
    Left = 0
    Top = 0
    Width = 458
    Height = 212
    HorzScrollBar.Smooth = True
    HorzScrollBar.Tracking = True
    Align = alClient
    BiDiMode = bdLeftToRight
    DoubleBuffered = True
    ParentBiDiMode = False
    ParentDoubleBuffered = False
    TabOrder = 0
    OnResize = ScrollBox1Resize
    ExplicitHeight = 337
    object Label1: TLabel
      Left = 192
      Top = 30
      Width = 69
      Height = 13
      BiDiMode = bdLeftToRight
      Caption = 'Details header'
      ParentBiDiMode = False
    end
    object Memo1: TMemo
      Left = 24
      Top = 70
      Width = 700
      Height = 89
      Lines.Strings = (
        'Details...')
      TabOrder = 0
    end
  end
end

I tried using DoubleBuffered but doesn't help.

Any suggestions how to make Label1 move without flickering/shaking, as smooth as Memo1 does when scrolling?

EDIT:

The design will eventually be that I have 3 or scrollboxes on form and each one contain up to 3 memos with header. And scrolling needs to be by scrollbox as all memos in same scroll box need to be scrolled at the same time. That means I do not see how it would work with putting label on form or panel and then on form, outside scrollboxes:

EDIT 2:

The answers below do provide good solutions, but they do make necessary to place the Labels, that are centered, out of the Scrollbox and put on the Form itself. And then move either by Scrollbox's scroll bars or by scroll bars directly on Form. This does get desired affect, but it adds a little inconvenience with Labels not being part of Scrollbox, anymore.

you do not have to place the label onto scrollbox. use a panel as container, place a label on top onto panel and place a scrollbox as client component onto panel. — sddk
You are aware that you don't need the scrollbox at all, because the memo will allow scrolling using it's own scrollbars if you turn them on (using the TMemo.Scrollbars property, ironically). If you use that built-in scrolling, you don't need the scrollbox at all, and your label never needs to move. — Ken White
@KenWhite, sddk: I can have up to 4 scrollboxes on form and up to 3 memos with headers in each scrollbox. So they need to be in grouped scrollboxes. I don't see how to put them on form/panel and to keep the needed design with scrolling relevant grouped information. See question edit. — Mike Torrettinni

Sertac Akyuz Sertac Akyuz · Accepted Answer · 2016-06-18T23:00:27

-" Memo moves smoothly, label doesn't."

That's because you're trying to prevent it from moving. Detach your OnScrollHorz handler and the label will move smoothly. But that's not what you want, it will not be centered to the form any more.

The problem is, during the inherited call (WM_HSCROLL), the label moves along with the memo. After the default handling, you relocate the label, hence the flicker.

You can expose an additional event handler that will fire before default scrolling (OnBeforeHorzScroll), and hide the label when it fires. While smoothly centered, it will cause a different kind of flicker where the label momentarily disappears. Still may not be satisfactory.

The solution is to use a control that is parented to the form, a sibling to the scroll box. You can't do that with a TLabel as it is a graphic control, but you can use TStaticText. The "structure pane" of the IDE may come handy if the static accidentally goes behind the scroll box at design time.

How to keep label smoothly centered in scrollbox?

3 Answers