[Qt-interest] Critical bug: QWidgets with size > 16383 break on OS X

[Nikos Chantziaras]

These macros are also defined on X11 and with same values (in 
src/gui/kernel/qwidget_x11.cpp) since the same limitation apply on X11 
too (and on MS Windows similar limitations apply.)


On 06/13/2010 03:46 PM, Constantin Makshin wrote:
> "src/gui/kernel/qwidget_mac.mm" has 2 lines that seem to be related to your problem:
>
> #define XCOORD_MAX 16383
> #define WRECT_MAX 8191
>
> Names of these macros and their usage make me think it's some sort of platform restriction Qt has to obey. I've never worked with MacOS so this is just a guess.
>
> On Sunday 13 June 2010 01:47:37 Nikos Chantziaras wrote:
>> I thought the maximum size of a QWidget in Qt4 was something like 2^24
>> (16 million-something) pixels on all platforms.  But on OS X, this isn't
>> the case.  I have a widget inside a QScrollView, and that widget can
>> grow very big in the Y (height) dimension (it's a text display where new
>> text is added at the bottom all the time.)  Heights over 40000 are not
>> uncommon.
>>
>> However, on Mac OS X, I'm busted.  When the widget grows over 16383
>> pixels in height, it wraps back to 8191, and my application gets very
>> messed up.  Note that QWIDGETSIZE_MAX is 16777215 on OS X too, so this
>> shouldn't happen.
>>
>> I will be reporting this to the bug tracker, but if someone knows of
>> some workaround for this, I would be very thankful, because right now,
>> the application is useless on the Mac :-/
>>
>> This happens with 4.6.2 and 4.7.0 beta1, both 32-bit.  Mac OS X 10.6.3
>> and 10.5.8.
>>
>> What follows is some example code that triggers the bug.  On Linux, this
>> shows a window that (once you scroll to the very bottom) prints "20000"
>> (the height of the widget).  On OS X, it prints "8191", which is the
>> height of the widget on the Mac, even though 20000 was set.
>>
>>
>> #include<QApplication>
>> #include<QScrollArea>
>> #include<QWidget>
>> #include<QPainter>
>>
>> class Widget: public QWidget
>> {
>> protected:
>>       void paintEvent( QPaintEvent* )
>>       {
>>           QPainter p(this);
>>           p.drawText(10, this->height(), QString::number(this->height()));
>>       }
>>
>> public:
>>       Widget(QWidget *parent = 0)
>>       : QWidget(parent)
>>       { }
>> };
>>
>>
>> int main(int argc, char *argv[])
>> {
>>       QApplication a(argc, argv);
>>       QScrollArea* scr = new QScrollArea;
>>       Widget* w = new Widget(scr);
>>       w->resize(500, 20000);
>>       scr->setWidget(w);
>>       scr->show();
>>       return a.exec();
>> }