Form disappeared

After I started working on dual monitors, I have found another interesting issue:

1. Create a new application with a main form and a sub-form.
2. Set the property Position of the sub-form to poMainFormCenter
3. Create a button "Show Sub-Form" on main form.
4. Move more than 50% area of the main form out of the main screen.
5. Click the button, we created.

The sub-form disappeared! Because the popup position of sub-form is out of the screen. I did a similar test with Internet Explorer. Its dialog will be shown at the corner of the screen. It can be regarded as a bug, so I have reported this issue to QualityCentral [1] as well.

Solution

This time you have to modify Forms.pas

1. Copy Forms.pas to your project folder,
2. find and replace with the following code,
3. and then recompile your project again.

{$IFDEF FIXUP_FORM_POPUP_POSITION}
function GetPrimaryMonitor: TMonitor;
var
  I: Integer;
begin
  for I := 0 to Screen.MonitorCount - 1 do
  begin
    Result := Screen.Monitors[I];
    if Result.Primary then
      Exit;
  end;
  Result := Screen.Monitors[0];
end;

procedure CenterFormToPrimaryMonitorCenter(var X, Y: Integer; 
  AForm: TCustomForm);
var
  R: TRect;
begin
  R := GetPrimaryMonitor.WorkareaRect;
  X := R.Left + (R.Right - R.Left - AForm.Width) div 2;
  Y := R.Top + (R.Bottom - R.Top - AForm.Height) div 2;
end;

procedure SnapFormToMonitorEdgeOnDemand(var X, Y: Integer; 
  AForm: TCustomForm);
var
  DistanceLeft, DistanceRight: Integer;
  CenterMon: TMonitor;
  R: TRect;
begin
  CenterMon := Screen.MonitorFromPoint(
    Point(X + AForm.Width div 2, Screen.DesktopHeight div 2));
  R := CenterMon.WorkareaRect;
  // Adjust X-pos
  DistanceLeft  := X - R.Left;
  DistanceRight := R.Right - (X + AForm.Width);
  if (DistanceLeft < 0) and (DistanceRight < 0) then
  begin
    if DistanceLeft >= DistanceRight then
         X := R.Left // Snap to left edge
    else X := R.Right - AForm.Width; // Snap to right edge
  end
  else if (DistanceLeft < 0) or (DistanceRight < 0) then
  begin
    if DistanceLeft < 0 then
         X := R.Left // Snap to left edge
    else X := R.Right - AForm.Width; // Snap to right edge
  end;
  // Adjust Y-pos
  if Y < R.Top then
    Y := R.Top
  else if Y > R.Bottom - AForm.Height then
    Y := R.Bottom - AForm.Height;
end;
{$ENDIF}

procedure TCustomForm.CMShowingChanged(var Message: TMessage);
// ...
    if (FPosition = poScreenCenter) or
       ((FPosition = poMainFormCenter) and (FormStyle = fsMDIChild)) then
    begin
      if FormStyle = fsMDIChild then
      begin
        X := (Application.MainForm.ClientWidth - Width) div 2;
        Y := (Application.MainForm.ClientHeight - Height) div 2;
      end else
      begin
      {$IFDEF FIXUP_FORM_POPUP_POSITION}
        CenterFormToPrimaryMonitorCenter(X, Y, Self);
      {$ELSE}
        X := (Screen.Width - Width) div 2;
        Y := (Screen.Height - Height) div 2;
      {$ENDIF}
      end;
    {$IFDEF FIXUP_FORM_POPUP_POSITION}
      SnapFormToMonitorEdgeOnDemand(X, Y, Self);
      SetBounds(X, Y, Width, Height);
      //SetWindowToMonitor() will cause unexpected popup position change!
    {$ELSE}
      if X < Screen.DesktopLeft then
        X := Screen.DesktopLeft;
      if Y < Screen.DesktopTop then
        Y := Screen.DesktopTop;
      SetBounds(X, Y, Width, Height);
      if Visible then SetWindowToMonitor;
    {$ENDIF}
    end
    else if FPosition in [poMainFormCenter, poOwnerFormCenter] then
    begin
      CenterForm := Application.MainForm;
      if (FPosition = poOwnerFormCenter) and (Owner is TCustomForm) then
        CenterForm := TCustomForm(Owner);
      if Assigned(CenterForm) then
      begin
        X := ((CenterForm.Width - Width) div 2) + CenterForm.Left;
        Y := ((CenterForm.Height - Height) div 2) + CenterForm.Top;
      end else
      begin
      {$IFDEF FIXUP_FORM_POPUP_POSITION}
        CenterFormToPrimaryMonitorCenter(X, Y, Self);
      {$ELSE}
        X := (Screen.Width - Width) div 2;
        Y := (Screen.Height - Height) div 2;
      {$ENDIF}
      end;
    {$IFDEF FIXUP_FORM_POPUP_POSITION}
      SnapFormToMonitorEdgeOnDemand(X, Y, Self);
      SetBounds(X, Y, Width, Height);
      //SetWindowToMonitor() will cause unexpected popup position change!
    {$ELSE}
      if X < Screen.DesktopLeft then
        X := Screen.DesktopLeft;
      if Y < Screen.DesktopTop then
        Y := Screen.DesktopTop;
      SetBounds(X, Y, Width, Height);
      if Visible then SetWindowToMonitor;
    {$ENDIF}
    end
    else if FPosition = poDesktopCenter then
// ...
end;

Conclusion

This issue is nothing. The motivation of this article [2] is to suggest you figure out the differences among DesktopRect, BoundsRect and WorkAreaRect. They are almost the same on one monitor environment. But they are totally different on dual monitors, especially when TaskBar is not at its default position.

References

1. Related discuss at QualityCentral.
2. The Chinese version of this article (on my blog @csdn)