为什么在MouseMove事件中调用WindowFromPoint时，窗体的系统按钮会突出显示？

Question

在TWinControl的MouseMove事件中调用WindowFromPoint会在传递给WindowFromPoint的点处导致MouseOver事件。这是一个VCL错误吗？有人知道是否有解决方法？

以下是演示代码：

unit Unit7;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  TForm7 = class(TForm)
    Button1: TButton;
    procedure Button1MouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form7: TForm7;

implementation

{$R *.dfm}

procedure TForm7.Button1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  WindowFromPoint(Point(Mouse.CursorPos.X, Mouse.CursorPos.Y - 40));
end;

end.

DFM：

object Form7: TForm7
  Left = 0
  Top = 0
  Caption = 'Form7'
  ClientHeight = 40
  ClientWidth = 116
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object Button1: TButton
    Left = 24
    Top = 7
    Width = 75
    Height = 25
    Caption = 'Button1'
    TabOrder = 0
    OnMouseMove = Button1MouseMove
  end
end

我在Windows 7 Pro 64bit上使用Delphi XE2。我也可以使用Delphi 7重现。

Answer 1

我用最简单的C ++应用程序对此进行了测试并观察到了相同的行为，这不是VCL错误（正如David在评论中提到的那样）。它与鼠标移动BTW无关，只要你通过标题按钮的坐标调用WindowFromPoint，就会出现特殊情况。它只发生在属于调用函数的线程的窗口上。

因此，对于解决方法，您可以从线程中调用WindowFromPoint。下面的简单示例，并非真正的后台线程，因为代码等待它完成：

type
  TGetWndThread = class(TThread)
  private
    FPoint: TPoint;
  protected
    procedure Execute; override;
    constructor Create(AOwner: TComponent; Point: TPoint);
  end;

constructor TGetWndThread.Create(AOwner: TComponent; Point: TPoint);
begin
  FPoint := Point;
  inherited Create;
end;

procedure TGetWndThread.Execute;
begin
  ReturnValue := WindowFromPoint(FPoint);
end;

..

var
  Wnd: HWND;
  Thr: TGetWndThread;
begin
  Thr := TGetWndThread.Create(nil, Point(Mouse.CursorPos.X, Mouse.CursorPos.Y - 40));
  Wnd := Thr.WaitFor;
  Thr.Free;
  .. // use Wnd

测试错误显示的条件（操作系统，主题..）并使代码有条件以避免不必要的开销是有意义的。