How to detect when the form is being maximized?

You can use the WM_GETMINMAXINFO message to save the state of the window and then use the WMSize message to check if the window was maximized.

in you form declare the mesage handler like so :

procedure WMSize(var Msg: TMessage); message WM_SIZE;

And handle like this :

procedure TForm57.WMSize(var Msg: TMessage);begin  if Msg.WParam  = SIZE_MAXIMIZED then    ShowMessage('Maximized');    end;

WIN+UP does not generate WM_SYSCOMMAND messages, that is why you cannot catch them. It does generate WM_GETMINMAXINFO, WM_WINDOWPOSCHANGING, WM_NCCALCSIZE, WM_MOVE, WM_SIZE, and WM_WINDOWPOSCHANGED messages, though. Like RRUZ said, use WM_GETMINMAXINFO to detect when a maximize operation is about to begin and WM_SIZE to detect when it is finished.

IMO, You cannot use WM_GETMINMAXINFO to "detect when a maximize operation is about to begin" as @Remy stated.

In-fact the only message that can is WM_SYSCOMMAND with Msg.CmdType=SC_MAXIMIZE or undocumented SC_MAXIMIZE2 = $F032 - but it's not being sent via Win+UP, or by using ShowWindow(Handle, SW_MAXIMIZE) for example.

The only way I could detect that a window is about to be maximized is via WM_WINDOWPOSCHANGING which is fired right after WM_GETMINMAXINFO:

type  TForm1 = class(TForm)  private    procedure WMWindowPosChanging(var Message: TWMWindowPosChanging); message WM_WINDOWPOSCHANGING;  end;implementationconst  SWP_STATECHANGED = $8000;procedure TForm1.WMWindowPosChanging(var Message: TWMWindowPosChanging);begin  inherited;  if (Message.WindowPos^.flags and (SWP_STATECHANGED or SWP_FRAMECHANGED)) <> 0 then  begin    if (Message.WindowPos^.x < 0) and (Message.WindowPos^.y < 0) then      ShowMessage('Window state is about to change to MAXIMIZED');  end;end;