Use the OnMouseActivate event handler to implement any special processing that should occur as a result of pressing a mouse button on a control when the parent top-level form is not active. 

The event type is:

TMouseActivateEvent = procedure (Sender: TObject; Button: TMouseButton; ShiftState: TShiftState; 
X, Y: Integer; HitTest: Integer; var MouseActivate: TMouseActivate) of object;

When you click on a control and the parent top-level form is not active, this event fires on the control that the mouse cursor is over. MouseActivate is a protected virtual function in TControl. MouseActivate can be overridden in custom control descendants to provide special built-in processing of the OnMouseActivate events. If you leave the default value of MouseActivate as maDefault, the parent control fires the OnMouseActivate event and repeats the process all the way up to the top-level form. If no control in the chain sets the value of MouseActivate, the behavior is the same as if a control set TMouseActivate to maActivate. At any point, a control can set the value of TMouseActivate and the parent processing stops.  

Some controls set focus to themselves when they get a button down message so, in some cases, setting maNoActivate appears to have no effect. For instance, a TButton control sets focus to itself in the WM_LBUTTONDOWN message regardless of whether or not TMouseActivate is set to maNoActivate. In this case, setting maNoActivateAndEat will work because the top-level form is not activated and the WM_LBUTTONDOWN message is suppressed.  

The HitTest parameter is the hit test value obtained from the WM_NCHITTEST message. See Windows.pas for valid values and their meanings. In most cases this value will be HTCLIENT, which means that the user clicked in the client area of a control. However, in the case of a top-level form, this can take other values such as HTCAPTION, or HTBOTTOM. This allows the code to decide which value to set for MouseActivate, based on the HitTest code. For example, the following code in the OnMouseActivate event handler for a TForm forces the user to click into a non-client area to activate the form:

if HitTest = HTCLIENT then
    MouseActivate := maNoActivateAndEat;

Even though this event is tied to how Windows processes WM_MOUSEACTIVATE, the actual handling of the message is done at the TControl level, which is the point in the VCL hierarchy where the control has no window handle. The OnMouseActivate event is then simulated since the all TControl descendants also have OnMouseDown, OnMouseMove, and OnMouseUp events. This information is applicable to both VCL Win32 and VCL.NET. 

OnClick 

OnMouseMove 

OnMouseUp 

MouseCapture