On Wed, Dec 31, 2025 at 08:09:21AM +0100, Adam Chyła wrote:
> Hi Dmitry,
>
> thank you for your reply.
>
> The 7238123521708ec9 commit will fix this issue, It would be great to have
> it backported to the Trixie.
>
> I think the issue is with the QXcbVirtualDesktop::dpi function (division by
> 0),
> which is also used in other places. The other code paths may still be
> affected.
>
> Other calls to QXcbVirtualDesktop::dpi:
>
> qtbase-everywhere-src-5.15.8/src/plugins/platforms/xcb/qxcbscreen.cpp
> 547 } else if (xineramaScreenInfo) {
> 548 m_geometry = QRect(xineramaScreenInfo->x_org,
> xineramaScreenInfo->y_org,
> 549 xineramaScreenInfo->width,
> xineramaScreenInfo->height);
> 550 m_availableGeometry =
> m_virtualDesktop->availableGeometry(m_geometry);
> 551 m_sizeMillimeters = sizeInMillimeters(m_geometry.size(),
> m_virtualDesktop->dpi()); // <-----
> 552 if (xineramaScreenIdx > -1)
> 553 m_outputName += QLatin1Char('-') +
> QString::number(xineramaScreenIdx);
> 554 }
> 555This block is for the case where RandR is not available, but Xinerama is available. I think it's quite a rare scenario nowadays. Qt 6 dropped support for Xinerama completely [1]. [1]: https://code.qt.io/cgit/qt/qtbase.git/commit?id=c91d1fdc100cda88 > ... > 799 // It can be that physical size is unknown while virtual size > 800 // is known (probably back-calculated from DPI and resolution), > 801 // e.g. on VNC or with some hardware. > 802 if (m_sizeMillimeters.isEmpty()) > 803 m_sizeMillimeters = sizeInMillimeters(geometry.size(), > m_virtualDesktop->dpi()); // <----- > > I see that in the newer Qt version, this function is only called from > the line 803. Yes, this code remains, but it is also a fallback code, I believe in normal scenarios m_sizeMillimeters.isEmpty() will be false. In case it causes problems, it would make sense to report a bug upstream with details. I will now cherry-pick 7238123521708ec9 and upload it first to unstable, then (if nobody complains after some time) to stable. -- Dmitry Shachnev
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