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 }
555
...
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.
Best regards,
Adam.
W dniu 27.12.2025 o 21:06, Dmitry Shachnev pisze:
Hi Adam, and sorry for the delayed reply! On Wed, Jun 04, 2025 at 10:20:04PM +0200, Adam Chyła wrote:The Qt-based applications fail to run. All the apps hang without displaying anything on the screen, then terminate. Setting the QT_FONT_DPI environment variable to a sensible default, such as 100, can be used as a workaround for the issue. [...] For this particular case, the platformScreen->logicalDpi() code calls the QXcbScreen::logicalDpi function: qtbase-everywhere-src-5.15.8/src/plugins/platforms/xcb/qxcbscreen.cpp 720 int QXcbScreen::forcedDpi() const 721 { 722 const int forcedDpi = m_virtualDesktop->forcedDpi(); 723 if (forcedDpi > 0) 724 return forcedDpi; 725 return 0; 726 } 727 728 QDpi QXcbScreen::logicalDpi() const 729 { 730 const int forcedDpi = this->forcedDpi(); 731 if (forcedDpi > 0) 732 return QDpi(forcedDpi, forcedDpi); 733 734 // Fall back to physical virtual desktop DPI, but prevent 735 // using DPI values lower than 96. This ensuers that connecting 736 // to e.g. a TV works somewhat predictabilly. 737 QDpi virtualDesktopPhysicalDPi = m_virtualDesktop->dpi(); 738 return QDpi(std::max(virtualDesktopPhysicalDPi.first, 96.0), 739 std::max(virtualDesktopPhysicalDPi.second, 96.0)); 740 }I believe this commit from Qt 6 should fix this issue, since it removes the call to dpi(): https://code.qt.io/cgit/qt/qtbase.git/commit?id=7238123521708ec9 Does this change look good to you? If yes, I will upload it to unstable, and if you want, I can also prepare a fix for Debian stable (Trixie). -- Dmitry Shachnev
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