windows: Fix vertical metrics with GDI engine

In Qt 6 we consolidated vertical font metrics across all
platforms (see f761ad3cd9ad1252f24b76ae413298dc7bed8af3
and follow-ups). However, a couple mistakes were made.

First of all, when we use the winAscent/winDescent values
from the OS/2 table, we would also set the leading to the
line gap value from the HHEA table. However, the line gap
is actually built into the winAscent/winDescent, so we
ended up adding this twice to the line spacing, increasing
it with some older fonts. When using the Windows legacy
metrics, we now set the line gap to 0 instead to reflect
that this is baked into the height.

In addition, since we now calculate the values ourselves,
we would not round them to nearest integer like the GDI
engine does. We now round these values to make it clear that
the GDI engine does not support any fractional metrics.

[ChangeLog][Windows][Text] Fixed an issue where the line
gap of some fonts would be included twice in the font's
leading.

Fixes: QTBUG-109400
Change-Id: I02ab7447b5e82d9f4474f9bca581f82acee85ff3
Reviewed-by: Qt CI Bot <[email protected]>
Reviewed-by: Lars Knoll <[email protected]>
(cherry picked from commit 8892819d0c89d0434c94f0c0951458719cba5c2e)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index 8b0dff31af7..6b764dee549 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/src/gui/text/qfontengine.cpp
@@ -416,6 +416,12 @@ void QFontEngine::initializeHeightMetrics() const
 
         // Allow OS/2 metrics to override if present
         processOS2Table();
+
+        if (!supportsSubPixelPositions()) {
+            m_ascent = m_ascent.round();
+            m_descent = m_descent.round();
+            m_leading = m_leading.round();
+        }
     }
 
     m_heightMetricsQueried = true;
@@ -448,6 +454,7 @@ bool QFontEngine::processOS2Table() const
                 return false;
             m_ascent = QFixed::fromReal(winAscent * fontDef.pixelSize) / unitsPerEm;
             m_descent = QFixed::fromReal(winDescent * fontDef.pixelSize) / unitsPerEm;
+            m_leading = QFixed{};
         }
 
         return true;