Author: Jonas Bilinkevicius

On a TRichEdit component a line can be selected by moving the cursor to the very 
left of the line (the cursor will change to a right pointing arrow) and click the 
mouse. It seems that the position of the cursor must be in the left most pixel 
(i.e. only one pixel wide) so is often difficult to use. I would like to extend 
this functionality to be available for the full width of the border, but I cannot 
find out where this functionality is implemented. It is not available on the TMemo 
component so I presume it must be implemented in the TCustomRichEdit component but 
I can't find it. Can some please point me in the right direction or any suggestions 
on how to extend this functionality to be applied to the full border width.

Answer:

First, the width of the area where the TRichEdit control allows you to select a 
whole line is defined by the formatting rectangle of the control. The difference 
between ClientRect left and formatting rectangle left coordinates normally is 1 
pixel, but you can move the left side of the formatting rectangle and the area 
where the cursor changes to a right pointing arrow and the RichEdit allows to 
select a line would be wider. You can do it both in the form OnCreate event and in 
the code of a TRichEdit descendant.

1   { ... }
2   var
3     XRect: TRect;
4   begin
5     SendMessage(MyRichEdit1.Handle, EM_GETRECT, 0, integer(@XRect));
6     XRect.Left := XRect.Left + 10; {set new left border for formatting rectangle}
7     SendMessage(MyRichEdit1.Handle, EM_SETRECT, 0, integer(@XRect));
8     { ... }


Regarding the selection of a whole line by clicking on the border, you should 
respond to the WM_NCHITTEST message and return a HTCLIENT constant in the message 
result in case the cursor is over border. The example below is a TRichEdit which 
has a LeftIndent property. By selecting it, you can set the left side of the 
formatting rectangle of the TRichEdit control.

9   { ... }
10  TMyRichEdit = class(TRichEdit)
11  protected
12    FLeftIndent: integer;
13    procedure WMNCHitTest(var message: TWMNCHitTest); message WM_NCHITTEST;
14    procedure SetEditRect;
15    procedure SetLeftIndent(AValue: integer);
16  public
17    constructor Create(AOwner: TComponent); override;
18  published
19    property LeftIndent: integer read FLeftIndent write SetLeftIndent;
20  end;
21  
22  { ... }
23  
24  constructor TMyRichEdit.Create(AOwner: TComponent);
25  begin
26    inherited Create(AOwner);
27    FLeftIndent := 0;
28  end;
29  
30  procedure TMyRichEdit.WMNCHitTest(var message: TWMNCHitTest);
31  begin
32    inherited;
33    if (message.Result = HTBORDER) then
34      message.Result := HTCLIENT;
35  end;
36  
37  procedure TMyRichEdit.SetEditRect;
38  var
39    XRect: TRect;
40  begin
41    SendMessage(Handle, EM_GETRECT, 0, integer(@XRect));
42    XRect.Left := FLeftIndent;
43    SendMessage(Handle, EM_SETRECT, 0, integer(@XRect));
44  end;
45  
46  procedure TMyRichEdit.SetLeftIndent(AValue: integer);
47  begin
48    if FLeftIndent <> AValue then
49    begin
50      FLeftIndent := AValue;
51      SetEditRect;
52    end;
53  end;