[fpc-devel] MDI problem and fix for Windows with Qt4/Qt5

Rudolf Cornelissen rudhaiku at gmail.com
Mon Sep 3 15:46:22 CEST 2018

Hi all,

This is my first post here, I hope my input is considered helpfull.

Anyhow, I am busy converting a Delphi MDI application (which I develop as a
daytime job) to Lazarus using Qt/Qt5.
Compared to Win32 Widgeset indeed Qt has much more stuff in place.

A problem I encountered is a mouse offset between mouseposition in the MDI
child window 'onscreen' and the position as handled internally in LCL when
a button in pressed.

The effect is that for instance if I have a button onscreen, and I hover
over it, the color correctly changes hinting me that I can press that
button. When I do however, the button is not activated unless I hover my
mouse 'titlebar height' above the button.

There is a simple fix possible for this my modifying the file
'qtwidgets.pas' in both Qt and Qt5 LCL/interfaces folders.

When I modify routine 'TQtMainWindow.OffsetMousePos' in the below way MDI
works fully correctly concerning the mouse positions here (windows7 tested):

procedure TQtMainWindow.OffsetMousePos(APoint: PQtPoint);
  MyWidget         : QWidgetH;
  MyStyle          : QStyleH;
  MyTitleBarHeight : Integer;
  inherited OffsetMousePos(APoint);

  if IsMdiChild and not IsMaximized then
    //correct windowed Y-pos
    //dec(APoint^.Y, GetSystemMetrics(SM_CYCAPTION)); //windows only
    MyWidget := GetWidget;
    MyStyle  := QWidget_style(MyWidget);
    MyTitleBarHeight := QStyle_pixelMetric(MyStyle
,QStylePM_TitleBarHeight, nil, MyWidget);
    dec(APoint^.Y, MyTitleBarHeight);

    //correct windowed X-pos
    dec(APoint^.X, (self.getFrameSize.Width -
self.getClientBounds.Size.Width) div 2);

Can someone validate I am not doing something stupid here and update the
sources if not?
That would save me some work manually modifying these routines over here..

Thanks for taking the time to look at this..

Kind regards,

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