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Add an envvar restoring Qt 5 behavior for vertical font metrics

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As a last resort for users who have existing Qt 5 applications and
see differences in line metrics when porting to Qt 6, we introduce
an environment variable which lets them restore the old behavior
as a last resort.

This is primarily meant as a porting tool, since we do have other
tools, such as QFont::PreferTypoLineMetrics to work around issues
with fonts that by mistake do not explicitly set the
use-typo-metrics flag even if these are valid.

Pick-to: 6.8
Fixes: QTBUG-134602
Change-Id: Id1d7829888d55247d2f07ee0a49b91ff34f26889
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
(cherry picked from commit 9a950b1ecb901cc885995d128768cc04cb55cfb1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 488e21860409b1f09b187c94e6e318f4ff8ff0a7)
This commit is contained in:
Eskil Abrahamsen Blomfeldt 2025-06-05 08:34:55 +02:00 committed by Qt Cherry-pick Bot
parent df7d38d7e1
commit 0e371a39ea
1 changed files with 8 additions and 1 deletions
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9
src/gui/text/qfontengine.cpp
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@ -421,7 +421,14 @@ void QFontEngine::initializeHeightMetrics() const
!getSfntTable(QFont::Tag("EBLC").value()).isEmpty()
|| !getSfntTable(QFont::Tag("CBLC").value()).isEmpty()
|| !getSfntTable(QFont::Tag("bdat").value()).isEmpty();
if (!hasEmbeddedBitmaps) {
// When porting applications from Qt 5 to Qt 6, users have noticed differences in line
// metrics due to the effort to consolidate these across platforms instead of using the
// system values directly. This environment variable gives a "last resort" for those users
// to tell Qt to prefer the metrics we get from the system, despite the fact these being
// inconsistent across platforms.
static bool useSystemLineMetrics = qEnvironmentVariableIntValue("QT_USE_SYSTEM_LINE_METRICS") > 0;
if (!hasEmbeddedBitmaps && !useSystemLineMetrics) {
// Get HHEA table values if available
processHheaTable();