tdf#153306 prevent subpixel shifting of X coordinate

HACK: for some unknown reason, if the X coordinate of the
path's bounds is more than 1024, SkBlendMode::kExclusion will
shift by about a half a pixel to the right with Skia/Metal on
a Retina display. Weirdly, if the same polygon is repeatedly
drawn, the total shift is cumulative so if the drawn polygon
is more than a few pixels wide, the blinking cursor in Writer
will exhibit this bug but only for one thin vertical slice at
a time. Apparently, shifting drawing a very tiny amount to
the left seems to be enough to quell this runaway cumulative
X coordinate shift.

Change-Id: Iabe9078e416b0dde003ee61c88817ff970cc8d39
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166768
Tested-by: Jenkins
Reviewed-by: Patrick Luby <guibomacdev@gmail.com>
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>

1 files changed, 20 insertions, 0 deletions

diff --git a/vcl/skia/gdiimpl.cxx b/vcl/skia/gdiimpl.cxx
index a6e030924ab6..eb6faeaa99d2 100644
--- a/vcl/skia/gdiimpl.cxx
+++ b/vcl/skia/gdiimpl.cxx
@@ -1489,6 +1489,26 @@ void SkiaSalGraphicsImpl::invert(basegfx::B2DPolygon const& rPoly, SalInvert eFl
             aPaint.setShader(
                 aBitmap.makeShader(SkTileMode::kRepeat, SkTileMode::kRepeat, SkSamplingOptions()));
         }
+
+#ifdef SK_METAL
+        // tdf#153306 prevent subpixel shifting of X coordinate
+        // HACK: for some unknown reason, if the X coordinate of the
+        // path's bounds is more than 1024, SkBlendMode::kExclusion will
+        // shift by about a half a pixel to the right with Skia/Metal on
+        // a Retina display. Weirdly, if the same polygon is repeatedly
+        // drawn, the total shift is cumulative so if the drawn polygon
+        // is more than a few pixels wide, the blinking cursor in Writer
+        // will exhibit this bug but only for one thin vertical slice at
+        // a time. Apparently, shifting drawing a very tiny amount to
+        // the left seems to be enough to quell this runaway cumulative
+        // X coordinate shift.
+        if (isGPU())
+        {
+            SkMatrix aMatrix;
+            aMatrix.set(SkMatrix::kMTransX, -0.25);
+            getDrawCanvas()->concat(aMatrix);
+        }
+#endif
     }
     getDrawCanvas()->drawPath(aPath, aPaint);
     postDraw();