Delphi’s T*Grid components have an annoying little feature whereby they will scroll the cell into view if you click on a partially visible cell at the right or the bottom of the window. Then, this couples with a timer that causes the scroll to continue as long as the mouse button is held down and the cell it is over is partially visible. This typically means that if a user clicks on a partially visible cell, they end up selecting a cell several rows or columns away from where they intended to click.

In my view, this is a bug that should be fixed in Delphi. I’m not the only person who thinks this. I’ve reported it to Embarcadero at RSP-18542.

In the meantime, here’s a little unit that works around the issue.

{
  Stop scroll on mousedown on bottom row of grid when bottom row
  is a partial cell: have to block both initial scroll and timer-
  based scroll.

  This code is pretty dependent on the implementation in Vcl.Grids.pas,
  so it should be checked if we upgrade to new version of Delphi.
}

{$IFNDEF VER320}
{$MESSAGE ERROR 'Check that this fix is still applicable for a new version of Delphi. Checked against Delphi 10.2' }
{$ENDIF}

unit ScrollFixedStringGrid;

interface

uses
  System.Classes,
  Vcl.Controls,
  Vcl.Grids,
  Winapi.Windows;

type
  TScrollFixedStringGrid = class(TStringGrid)
  private
    TimerStarted: Boolean;
    HackedMousedown: Boolean;
  protected
    procedure MouseDown(Button: TMouseButton; Shift: TShiftState; X: Integer;
      Y: Integer); override;
    procedure MouseMove(Shift: TShiftState; X: Integer; Y: Integer); override;
    function SelectCell(ACol, ARow: Longint): Boolean; override;
  end;

implementation

{ TScrollFixedStringGrid }

procedure TScrollFixedStringGrid.MouseDown(Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  // When we first mouse-down, we know the grid has
  // no active scroll timer
  TimerStarted := False;

  // Call the inherited event, blocking the default MoveCurrent
  // behaviour that scrolls the cell into view
  HackedMouseDown := True;
  try
    inherited;
  finally
    HackedMouseDown := False;
  end;

  // Cancel scrolling timer started by the mousedown event for selecting
  if FGridState = gsSelecting then
    KillTimer(Handle, 1);
end;

procedure TScrollFixedStringGrid.MouseMove(Shift: TShiftState; X, Y: Integer);
begin
  // Start the scroll timer if we are selecting and mouse
  // button is down, on our first movement with mouse down
  if not TimerStarted and (FGridState = gsSelecting) then
  begin
    SetTimer(Handle, 1, 60, nil);
    TimerStarted := True;
  end;
  inherited;
end;


function TScrollFixedStringGrid.SelectCell(ACol, ARow: Longint): Boolean;
begin
  Result := inherited;
  if Result and HackedMousedown then
  begin
    // MoveColRow calls MoveCurrent, which
    // calls SelectCell. If SelectCell returns False, then
    // movement is blocked. But we fake it by re-calling with Show=False
    // to get the behaviour we want
    HackedMouseDown := False;
    try
      MoveColRow(ACol, ARow, True, False);
    finally
      HackedMouseDown := True;
    end;
    Result := False;
  end;
end;

end.