Delphi – Taskbar width, height and position

Like so:

MainForm.Width := 127;
MainForm.Height := 263;

Or perhaps you want to set the client area to those dimensions:

MainForm.ClientWidth := 127;
MainForm.ClientHeight := 263;

Of course, you most commonly set these properties in the Object Inspector at design time and then they are written to your form's .dfm file.

If you want such a change to occur on a button click add a handler for the button click that looks like this:

procedure TMainForm.Button1Click(Sender: TObject);
begin
  Width := 127;
  Height := 263;
end;

In this last excerpt you don't need to specify the MainForm object instance because the event handler is a member of the TMainForm class and so the Self is implicit.

If you wish to follow Ulrich Gerhardt's advice (see comment) and use SetBounds then you would write:

SetBounds(Left, Top, 127, 263);

Finally, if your form has Scaled = True then you need to deal with font scaling. Hard coded pixel dimensions like this will not be appropriate for machines with font scaling set to a different value from your machine.

Ok, now that it is clear what you want, first a couple of things:

• The first Form created is automatically the MainForm,
• An application cannot without a MainForm; when the MainForm closes, the application closes, whatever other forms are shown,
• You can hide the MainForm,
• By default (in older Delphi versions anyway) the application's window is shown on the taskbar (Application.MainFormOnTaskbar = False). As long as the application is active, and as long as there is at least one form showing, this icon/window is shown in the taskbar.
• When Application.MainFormOnTaskbar = True, then the icon/window of the MainForm is shown in the taskbar. When the MainForm is hiden, the icon dissapears. Showing another form does not result in another taskbar icon/window, so there is no icon at all then.

So, it is clear you need to set Application.MainFormOnTaskbar := False in the project file.

Furthermore, the following combination of methods seems to work as you want:

Unit1/Form1/MainForm:

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

procedure TForm1.OpenForm2ButtonClick(Sender: TObject);
begin
  TForm2.Create(Self).Show;
  Hide;
end;

Unit2/Form2:

procedure TForm2.CloseButtonClick(Sender: TObject);
begin
  Close;
end;

procedure TForm2.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  TForm(Owner).Show;
  Action := caFree;
end;

procedure TForm2.OpenForm3ButtonClick(Sender: TObject);
begin
  TForm3.Create(Self).Show;
  Hide;
end;

Unit3/Form3:

procedure TForm3.CloseButtonClick(Sender: TObject);
begin
  Close;
end;

procedure TForm3.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  TForm(Owner).Show;
  Action := caFree;
end;

Note that the caption of the taskbar button remains the same during these changes. If you want to synchronize that with the caption of the form being shown, set Application.Title.