Port default font resolve to SystemParametersInfoForDpi

We were getting font metrics for the primary screen and then adjusting
for the screen DPI get a screen-independent font size.

This could fail in edge cases where the screen DPI was changed after
app startup, but before the first window was shown. See QTBUG-105857.

Use SystemParametersInfoForDpi() to query for font info at 96 DPI instead,
which removes the need for tracking primary screen DPI.

LOGFONT_to_QFont() still has one usage (qwizard_win.cpp) which provides
a custom DPI for scaling, so we keep that function as-is.

Fixes: QTBUG-105857
Change-Id: I1adf0ab3bf2c309e8fcb58093e86214fa11a2da8
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit edb00660e47a82ac24048841100d7d7e59a7805e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 4 insertions, 11 deletions

diff --git a/src/gui/text/windows/qwindowsfontdatabasebase.cpp b/src/gui/text/windows/qwindowsfontdatabasebase.cpp
index d05410cf5f..f45678c65c 100644
--- a/src/gui/text/windows/qwindowsfontdatabasebase.cpp
+++ b/src/gui/text/windows/qwindowsfontdatabasebase.cpp
@@ -566,16 +566,9 @@ void QWindowsFontDatabaseBase::createDirectWriteFactory(IDWriteFactory **factory
 }
 #endif // directwrite && direct2d
 
-static int s_defaultVerticalDPI = 96; // Native Pixels
-
 int QWindowsFontDatabaseBase::defaultVerticalDPI()
 {
-    return s_defaultVerticalDPI;
-}
-
-void QWindowsFontDatabaseBase::setDefaultVerticalDPI(int d)
-{
-    s_defaultVerticalDPI = d;
+    return 96;
 }
 
 LOGFONT QWindowsFontDatabaseBase::fontDefToLOGFONT(const QFontDef &request, const QString &faceName)
@@ -690,9 +683,9 @@ HFONT QWindowsFontDatabaseBase::systemFont()
 QFont QWindowsFontDatabaseBase::systemDefaultFont()
 {
     // Qt 6: Obtain default GUI font (typically "Segoe UI, 9pt", see QTBUG-58610)
-    NONCLIENTMETRICS ncm;
-    ncm.cbSize = FIELD_OFFSET(NONCLIENTMETRICS, lfMessageFont) + sizeof(LOGFONT);
-    SystemParametersInfo(SPI_GETNONCLIENTMETRICS, ncm.cbSize , &ncm, 0);
+    NONCLIENTMETRICS ncm = {};
+    ncm.cbSize = sizeof(ncm);
+    SystemParametersInfoForDpi(SPI_GETNONCLIENTMETRICS, ncm.cbSize, &ncm, 0, defaultVerticalDPI());
     const QFont systemFont = QWindowsFontDatabase::LOGFONT_to_QFont(ncm.lfMessageFont);
     qCDebug(lcQpaFonts) << __FUNCTION__ << systemFont;
     return systemFont;