Author: Jonas Bilinkevicius

How can I control where text is dropped into a TMemo? In other words, I am in the 
middle of a drag operation and want to drop selected text into a TMemo based on the 
mouse position of where I drop. How can I tell the caret to go to the mouse 
location prior to the drop action?

Answer:

Send a EM_CHARFROMPOS message to the control, passing the mouse position (in client 
coordinates).
1   
2   procedure TForm1.Memo1MouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
3   var
4     ret: Longint;
5   begin
6     ret := memo1.perform(EM_CHARFROMPOS, 0, MakeLParam(X, Y));
7     label1.caption := format('row: %d, character index: %d', [HiWord(ret),
8       LoWord(ret)]);
9   end;


That is the first step, it gives you the mouse position in "character coordiantes". 
You still need to convert that to a character index, which you assign to SelStart 
to set the caret to that position.

10  with memo1 do
11    selstart := perform(EM_LINEINDEX, row, 0) + col;


You can now assign the dropped text to the memos SelText property to insert it.

Note that the EM_CHARFROMPOS message can also be used with TRichedit but the 
parameters
are different!