unit Unit2;

interface

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

type
  TForm2 = class(TForm)
    Timer1: TTimer;
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    Label4: TLabel;
    Label5: TLabel;
    Label6: TLabel;
    Label7: TLabel;
    Label8: TLabel;
    Label9: TLabel;
    Label10: TLabel;
    Label11: TLabel;
    Label12: TLabel;
    procedure Timer1Timer(Sender: TObject);

  private
    { Private declarations }
  public
    { Public declarations }
    selectr:trect;          //注意这里定义为保存选定窗体的左上角
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

procedure TForm2.Timer1Timer(Sender: TObject);

var
  p: TPoint;
  h: HWND;
  r:trect;
begin
  GetCursorPos(p);
  label6.Caption:=format('x: %d, y: %d',[p.x,p.y]);  //鼠标在屏幕绝对位置
  h := WindowFromPoint(p);
  GetWindowRect(h,r);//得到窗口的左上角坐标

   if GetAsyncKeyState(VK_LButton)<>0 then  //按下鼠标左键，记下所选的窗体左上角
   begin
    selectr:=r;
    label11.Caption:=format('x: %d, y: %d',[r.left,r.top]);
   end;

  label4.Caption:=format('x: %d, y: %d',[r.left,r.top]); //当前鼠标位置窗体的左上角
  label8.Caption:=format('x: %d, y: %d',[p.x-selectr.left,p.y-selectr.top]);
  //label8内为发送鼠标事件所用数据
  Windows.ScreenToClient(h, p);
  label1.Caption:=format('x: %d, y: %d',[p.X,p.y]);//鼠标在窗口里的相对位置坐标

end;

end.

http://cy03wqh.blog.163.com/blog/static/3678030220108592514544/