Delphi, How to show an overlayed control on mouse move - delphi

Question

I use Delphi 7 and I have a TFrame (hosted by a TForm) with three panels that span over the whole surface, in a "upside down T" layout.
The panels should be resizeable, so I could use 2 splitters, but I want to give a better user experience: I'd like to have a single "size grip" in the T junction.
This "handle" should appear only when the user hovers the junction area.
So here is my question: what is the best way to have a control show on top of any other on mouse move?
TFrame.OnMouseMove don't get called (obviously) because there are the panels inside and possibly any sort of other controls inside them.
I also strongly want to keep all the code inside the frame.
I see 2 solutions:
Install a local Mouse Hook and go with it. But there could be some
performance issues (or not?)
Handle TApplication.OnMessage inside
the frame, but adding some other code in order to simulate a "chain"
of event handlers. This is because other parts of the application
could need to handle TApplication.OnMessage for their own purposes.
Any other idea?
Thank you

Answer 1

To make a mouse move event notifier for the whole frame, no matter which child control is hovered, you can write a handler for the WM_SETCURSOR message as I've learnt from TOndrej in this post. From such event handler you can then determine which control is hovered and bring it to front.
Please note, I have done quite commonly used mistake here. The GetMessagePos result must not be read this way. It's even explicitly mentioned in docs. I don't have Windows SDK to see the MAKEPOINTS macro, so I'll fix this later:
type
TFrame1 = class(TFrame)
// there are many controls here; just pretend :-)
private
procedure WMSetCursor(var Msg: TWMSetCursor); message WM_SETCURSOR;
end;
implementation
procedure TFrame1.WMSetCursor(var Msg: TWMSetCursor);
var
MsgPos: DWORD;
Control: TWinControl;
begin
inherited;
MsgPos := GetMessagePos;
Control := FindVCLWindow(Point(LoWord(MsgPos), HiWord(MsgPos)));
if Assigned(Control) then
Control.BringToFront;
end;

Answer 2

I'll post this self-answer just because it works and it could be useful in some cases, but I marked TLama's as the best answer.
This is the solution 2) of the question:
TMyFrame = class(TFrame)
// ...design time stuff...
private
FMouseHovering: Boolean;
FPreviousOnAppMessage: TMessageEvent;
procedure DoOnAppMessage(var Msg: TMsg; var Handled: Boolean);
protected
procedure CMMouseEnter(var Message: TMessage); message CM_MOUSEENTER;
procedure CMMouseLeave(var Message: TMessage); message CM_MOUSELEAVE;
public
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
end;
implementation
constructor TMyFrame.Create(AOwner: TComponent);
begin
inherited;
FMouseHovering := False;
FPreviousOnAppMessage := Application.OnMessage;
Application.OnMessage := DoOnAppMessage;
end;
destructor TMyFrame.Destroy;
begin
Application.OnMessage := FPreviousOnAppMessage;
inherited;
end;
procedure TRiascoFrame.CMMouseEnter(var Message: TMessage);
begin
FMouseHovering := True;
end;
procedure TRiascoFrame.CMMouseLeave(var Message: TMessage);
begin
FMouseHovering := False;
end;
procedure TMyFrame.DoOnAppMessage(var Msg: TMsg; var Handled: Boolean);
begin
if (Msg.message = WM_MOUSEMOVE) and FMouseHovering then
DoHandleMouseMove(Msg.hwnd, Integer(LoWord(Msg.lParam)), Integer(HiWord(Msg.lParam)));
if Assigned(FPreviousOnAppMessage) then
FPreviousOnAppMessage(Msg, Handled);
end;
procedure TMyFrame.DoHandleMouseMove(hWnd: HWND; X, Y: Integer);
var
ClientPoint: TPoint;
begin
ClientPoint := Point(X, Y);
Windows.ClientToScreen(hwnd, ClientPoint);
Windows.ScreenToClient(Self.Handle, ClientPoint);
if PtInRect(ClientRect, ClientPoint) then
begin
// ...do something...
end;
end;