Keep forms snapped together

If you want to do a multiple program form docking, I would set up some kind of interprocess communication, I will use a Memory Mapped file and WM_USER messages. Here is some code for a program that will run several times, each time you run the program it will produce a form that will log into a Memory Mapped file an place it's form handle there, there is also an "Info" member that will be set to One if the form is to be docked.
My form's width and height are small, and there is One TCheckBox named "CheckBoxDock" which is checked if the form os to Dock, and unchecked if NO docking. . . and two TButtons, one named "but_Close" which will just close that form, and one named  "but_CloseAll" which will close all of the Forms logged in the Mem mapped file. There is a TLabel, which just give some information. I have a TDockRec type that is the information for the forms in the mem mapped file. There is also a  TaryDockRec  type that is an Array to hold up to 9 forms information, I limited this to 9 forms, just to keep it easy.
There is a  WM_MOVING  message where i do the  DeferWindowPos( ) thing for all of the windows in the mem mapped array.
I did not have time to put many comments in this to tell you what it is doing, ask questions if you need more information - -




unit DockForms;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, Buttons;

type
  PDockRec = ^TDockRec;
  TDockRec = packed record
    Handle: THandle;
    Info: Cardinal;
    //OffSet: TPoint;
  // you can include an offSet point if you want to
    end;

  ParyDockRec = ^TaryDockRec;
  TaryDockRec = Array[0..8] of TDockRec;

  TForm1 = class(TForm)
    CheckBoxDock: TCheckBox;
    but_Close: TButton;
    Label1: TLabel;
    but_CloseAll: TButton;
    procedure but_CloseClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure CheckBoxDockClick(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure but_CloseAllClick(Sender: TObject);
  private
    { Private declarations }
    hMemFile: Integer;
    ParyDR1: ParyDockRec;
  // this ParyDR1 is a memMapped file for an array to keep the handles
  // of all the form windows in
    aryOffSets: Array[0..8] of TPoint;
  // this aryOffSets has 9 points for the position of each form
    function FindIndex: Integer;
    procedure WMMoving(var Msg: TMessage); message WM_MOVING;
    procedure ClosedMsg(var Msg: TMessage); message WM_USER+123;
  // Closed message is sent whenever a Form is destroyed
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}


procedure TForm1.ClosedMsg(var Msg: TMessage);
var
aLeft, aTop: SmallInt;
aRect: TRect;
i: Integer;
begin
if (ParyDR1 <> nil) and (ParyDR1[0].Handle = Handle) then
  if Msg.WParam = 555 then
    begin
    if ParyDR1[0].Info > 0 then
      begin
      GetWindowRect(Handle, aRect);
      for i := 1 to ParyDR1[0].Info do
        if ParyDR1[i].Info = 1 then
        MoveWindow(ParyDR1[i].Handle, aRect.Left+aryOffSets[i].x,
                   aRect.Top+aryOffSets[i].y, Width, Height, True);
      end;
    end else
    if Msg.Wparam = 3456 then
      begin
      CheckBoxDock.Hide;
      but_CloseAll.Show;
      aTop := SmallInt(Msg.LParam and $FFFF);
      aLeft := SmallInt((Msg.LParam and $FFFF0000) shr 16);
      MoveWindow(Handle, aLeft, aTop, Width, Height, True);
      if ParyDR1[0].Info > 0 then
        for i := 1 to ParyDR1[0].Info do
          if ParyDR1[i].Info = 1 then
          MoveWindow(ParyDR1[i].Handle, aLeft+aryOffSets[i].x,
                     aTop+aryOffSets[i].y, Width, Height, True);
      end;
end;


procedure TForm1.WMMoving(var Msg: TMessage);
var
Index, i, Hdwp1: Integer;
begin
if ParyDR1 <> nil then
  begin
  if ParyDR1[0].Info  > 0 then
    begin
    Index := FindIndex;
    if Index > -1 then
      begin
      if ParyDR1[Index].Info > 0 then
        begin
        Hdwp1 := BeginDeferWindowPos(ParyDR1[0].Info+1);
        for i := 0 to ParyDR1[0].Info do
          begin
          if (i > 0) and (ParyDR1[i].Info = 0) then Continue;
          DeferWindowPos(Hdwp1, ParyDR1[i].Handle, HWND_BOTTOM,
                         PRect(Msg.LParam)^.Left+aryOffSets[i].x-aryOffSets[Index].x,
          PRect(Msg.LParam)^.Top+aryOffSets[i].y-aryOffSets[Index].y, 1, 1,
          SWP_NOSIZE or SWP_NOZORDER or SWP_NOACTIVATE);
          end;
        EndDeferWindowPos(Hdwp1);
        Exit;
        end;
      end;
    end;

  end;
inherited;
end;

procedure TForm1.but_CloseClick(Sender: TObject);
begin
Close;
end;

procedure TForm1.FormCreate(Sender: TObject);
var
LErr: Integer;
begin
{I did not even try to do a drag and drop Dock operation,
because of the math I would need to for that, I just Dock these
forms in a position relative to their array index number, it
is more easy to do

the aryOffSets holds the 9 positions of each form as an offset to
the Left and Top of the FIRST form}
aryOffSets[0].x := 0;
aryOffSets[0].y := 0;

aryOffSets[1].x := 0; // under first
aryOffSets[1].y := Height;

aryOffSets[2].x := Width;
aryOffSets[2].y := 0; // right side of first

aryOffSets[3].x := 0; // above first
aryOffSets[3].y := -Height;

aryOffSets[4].x := -Width;
aryOffSets[4].y := 0; // left side of first

aryOffSets[5].x := -Width;
aryOffSets[5].y := Height; // left and under first

aryOffSets[6].x := Width;
aryOffSets[6].y := Height; // right and under first

aryOffSets[7].x := Width;
aryOffSets[7].y := -Height; // right and above first

aryOffSets[8].x := -Width;
aryOffSets[8].y := -Height; // left and above first

hMemFile := CreateFileMapping(MaxDWord, nil, PAGE_READWRITE, 0,
                SizeOf(TaryDockRec), 'yU7B8L+A8?');

LErr := GetLastError;
ParyDR1 := MapViewOfFile(hMemFile, FILE_MAP_WRITE, 0, 0, 0);
if ParyDR1 = nil then
  Label1.Caption := 'Docking NOT Availible'
  else
  if LErr = 0 then
    begin
    CheckBoxDock.Hide;
    but_CloseAll.Show;
    ZeroMemory(ParyDR1, SizeOf(TaryDockRec));
    ParyDR1[0].Handle := Handle;
    ParyDR1[0].Info := 0;
    //ParyDR1[0].OffSet.x := 0;
    //ParyDR1[0].OffSet.y := 0;
    Label1.Caption := 'FIRST Form  Form Handle '+IntToStr(Handle);
    end else
    begin
    but_CloseAll.Hide;
    if ParyDR1[0].Info = 8 then
      begin
      CheckBoxDock.Hide;
      Exit;
      end;
    Inc(ParyDR1[0].Info);
    ParyDR1[ParyDR1[0].Info].Handle := Handle;
    ParyDR1[ParyDR1[0].Info].Info := 0;
    //ParyDR1[ParyDR1[0].Info].OffSet.x := yourXoffset;
    //ParyDR1[ParyDR1[0].Info].OffSet.y := yourYoffset;
  // if you want to try and do a drag Dock, you may need these OffSet values
    Label1.Caption := 'Form number '+IntToStr(ParyDR1[0].Info)+'  Form Handle '+
                      IntToStr(Handle);
    end;

end;


function TForm1.FindIndex: Integer;
var
i: Integer;
begin
Result := -1;
if ParyDR1 = nil then
  Exit;

if ParyDR1[0].Info = 0 then
  begin
  Result := 0;
  Exit;
  end;
for i := 0 to ParyDR1[0].Info do
   if ParyDR1[i].Handle = Handle then
     begin
     Result := i;
     Break;
     end;
end;


procedure TForm1.CheckBoxDockClick(Sender: TObject);
var
Index: Integer;
aRect: TRect;
begin
Index := FindIndex;
if Index < 1 then
  begin
  CheckBoxDock.Hide;
  Exit;
  end;
if CheckBoxDock.Checked then
  begin
  ParyDR1[Index].Info := 1;
  GetWindowRect(ParyDR1[0].Handle, aRect);
  MoveWindow(Handle, aRect.Left+aryOffSets[Index].x,
             aRect.Top+aryOffSets[Index].y, Width, Height, True);
  end else
  ParyDR1[Index].Info := 0;

end;

procedure TForm1.FormDestroy(Sender: TObject);
var
Hold: Cardinal;
Index, sendI: Integer;
aRect: TRect;
begin
if ParyDR1 <> nil then
  begin
  if ParyDR1[0].Info > 0 then
    Hold := ParyDR1[0].Info-1
    else
    Hold := 0;
  Index := FindIndex;
  if Index > -1 then
    begin
    if Index = Integer(ParyDR1[0].Info) then
      begin
      ParyDR1[Index].Handle := 0;
      if Index > 0 then
        Dec(ParyDR1[0].Info);
      end else
      begin
      MoveMemory(@ParyDR1[Index], @ParyDR1[Index+1],
                 (ParyDR1[0].Info - Cardinal(Index)) * SizeOf(TDockRec));
      ParyDR1[Hold+1].Handle := 0;
      ParyDR1[0].Info := Hold;
      end;
   Sleep(10);
   if (Index = 0) and (ParyDR1[0].Handle <> 0) then
     begin
     GetWindowRect(Handle, aRect);
     sendI := (Word(SmallInt(aRect.Left)) shl 16) or Word(SmallInt(aRect.Top));
     PostMessage(ParyDR1[0].Handle, WM_USER+123, 3456, sendI);
     end;
   if Index > 0 then
     PostMessage(ParyDR1[0].Handle, WM_USER+123, 555, 0);
   end;
  end;
UnmapViewOfFile(ParyDR1);
CloseHandle(hMemFile);
end;

procedure TForm1.but_CloseAllClick(Sender: TObject);
var
i: Integer;
begin
for i := ParyDR1[0].Info DownTo 0 do
  PostMessage(ParyDR1[i].Handle, WM_CLOSE, 0, 0);
end;

end.


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I did not have time to fully test this , but it seems to work for me. . .  it should give you some ideas on a way to try and do what you have described