[Webkit-unassigned] [Bug 246100] Text is slightly misaligned at specific font sizes or container widths

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=246100

--- Comment #6 from zalan <zalan at apple.com> ---
This seems to be caused by the lack of inline content (horizontal) pixelsnapping (the visual glitch shows up when the block painting produces a pixelsnapped (position/size) block box while the inline painting stays on a non-snapped position. On a 2x display, it's most visible at 0.25 and 0.75 where the block box get pixelsnapped to 0.5, 1, while the inline content is still painted at 0.25 and 0.75 (touching border).

the following snippet fixes the test case (it surely needs some more work though).

diff --git a/Source/WebCore/rendering/RenderBlockFlow.cpp b/Source/WebCore/rendering/RenderBlockFlow.cpp
index 6f46a2aaf2ae..0141ac14dfff 100644
--- a/Source/WebCore/rendering/RenderBlockFlow.cpp
+++ b/Source/WebCore/rendering/RenderBlockFlow.cpp
@@ -3486,13 +3486,14 @@ void RenderBlockFlow::paintInlineChildren(PaintInfo& paintInfo, const LayoutPoin
 {
     ASSERT(childrenInline());

+    auto adjustedPaintOffset = LayoutPoint { roundPointToDevicePixels(paintOffset, document().deviceScaleFactor()) };
     if (modernLineLayout()) {
-        modernLineLayout()->paint(paintInfo, paintOffset);
+        modernLineLayout()->paint(paintInfo, adjustedPaintOffset);
         return;
     }

     if (legacyLineLayout())
-        legacyLineLayout()->lineBoxes().paint(this, paintInfo, paintOffset);
+        legacyLineLayout()->lineBoxes().paint(this, paintInfo, adjustedPaintOffset);
 }

 bool RenderBlockFlow::relayoutForPagination()

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