how do I avoid this unwanted behaviour with Delphi's TSplitter and panels?

Question

Included is a small project demonstrating my problem. I have a TPageControl aligned to the main form. On each of two tabsheets I have panels client aligned. On each of those panels I have 2 subpanels and a splitter. The LH panel and splitter is aligned left, the RH panel client-aligned.

Basically the problem is interaction between the 2 tabs. To demonstrate:

• run the program
• stretch the main form horizontally. Panel 3 will grow
• move the splitter as far to the right as it will go. Panel 2 will grow, Panel 3 will shrink to its 10-pixel min-width constraint.
• select tabsheet 2. Panel 5 is as designed, panel 6 grew when the main form was stretched
• reduce the main form width to its original width. Panel 6 shrinks too much (undesirable)
• click on tabsheet 1. Main form increases in width again (undesirable)

OK, the behaviour is probably explainable in terms of the rules of aligned panels, but can anybody suggest improvements to the operation?

unit Unit17;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, ComCtrls;

type
  TForm17 = class(TForm)
    PageControl1: TPageControl;
    TabSheet1: TTabSheet;
    TabSheet2: TTabSheet;
    Panel1: TPanel;
    Panel2: TPanel;
    Splitter1: TSplitter;
    Panel3: TPanel;
    Panel4: TPanel;
    Splitter2: TSplitter;
    Panel5: TPanel;
    Panel6: TPanel;
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form17: TForm17;

implementation

{$R *.dfm}

end.


object Form17: TForm17
  Left = 0
  Top = 0
  Caption = 'Form17'
  ClientHeight = 254
  ClientWidth = 314
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object PageControl1: TPageControl
    Left = 0
    Top = 0
    Width = 314
    Height = 254
    ActivePage = TabSheet1
    Align = alClient
    Constraints.MinWidth = 30
    TabOrder = 0
    ExplicitWidth = 480
    object TabSheet1: TTabSheet
      Caption = 'TabSheet1'
      ExplicitWidth = 281
      ExplicitHeight = 165
      object Panel1: TPanel
        Left = 0
        Top = 0
        Width = 306
        Height = 226
        Align = alClient
        Caption = 'Panel1'
        TabOrder = 0
        ExplicitWidth = 109
        ExplicitHeight = 165
        object Splitter1: TSplitter
          Left = 151
          Top = 1
          Width = 12
          Height = 224
          ExplicitLeft = 145
        end
        object Panel2: TPanel
          Left = 1
          Top = 1
          Width = 150
          Height = 224
          Align = alLeft
          Caption = 'Panel2'
          Constraints.MinWidth = 10
          TabOrder = 0
        end
        object Panel3: TPanel
          Left = 163
          Top = 1
          Width = 142
          Height = 224
          Align = alClient
          Caption = 'Panel3'
          Constraints.MinWidth = 10
          TabOrder = 1
          ExplicitLeft = 141
          ExplicitWidth = 330
        end
      end
    end
    object TabSheet2: TTabSheet
      Caption = 'TabSheet2'
      ImageIndex = 1
      ExplicitWidth = 281
      ExplicitHeight = 165
      object Panel4: TPanel
        Left = 0
        Top = 0
        Width = 306
        Height = 226
        Align = alClient
        Caption = 'Panel4'
        TabOrder = 0
        ExplicitWidth = 109
        ExplicitHeight = 165
        object Splitter2: TSplitter
          Left = 149
          Top = 1
          Width = 11
          Height = 224
          ExplicitLeft = 141
        end
        object Panel5: TPanel
          Left = 1
          Top = 1
          Width = 148
          Height = 224
          Align = alLeft
          Caption = 'Panel5'
          Constraints.MinWidth = 10
          TabOrder = 0
        end
        object Panel6: TPanel
          Left = 160
          Top = 1
          Width = 145
          Height = 224
          Align = alClient
          Caption = 'Panel6'
          Constraints.MinWidth = 10
          TabOrder = 1
          ExplicitLeft = 141
          ExplicitWidth = 139
          ExplicitHeight = 163
        end
      end
    end
  end
end 

Answer 1

To get expected behavior remove the constraints (MinWidth) from your panels. These settings are currently ineffective anyway, since your splitters have a MinSize of '30' (the default, not stored).

edit (response to the comment): You cannot expect the 'MinWidth' constraint of a control that's at the right side of a splitter to adjust the size of the left-side control. That's only logical, the constraint is a property for the control that you set. All you'll achieve is that the form will deny shrinking if your control is already at its 'MinWidth', hence the undesirable behavior that you observe that the form is getting larger when you switch tabs. What you desire, you have to do with code - as Marjan told in his answer. There should be more than one way to achieve this, for instance, put the below to the Panel3's 'OnCanResize' event:

procedure TForm1.Panel3CanResize(Sender: TObject; var NewWidth,
  NewHeight: Integer; var Resize: Boolean);
begin
  if NewWidth < Splitter1.MinSize then
    Panel2.Width := Panel2.Width - Splitter1.MinSize + NewWidth;
end;