Adjust visual overflow rect for rounded/shifted ascent/descent

third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp

 /*
  * Copyright (C) 2005, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 115 matching lines @@
   float ascent;
   float descent;
 
   // Beware those who step here: This code is designed to match Win32 font
   // metrics *exactly* except:
   // - the adjustment of ascent/descent on Linux/Android
   // - metrics.fAscent and .fDesscent are not rounded to int for tiny fonts
   if (isVDMXValid) {
     ascent = vdmxAscent;
     descent = -vdmxDescent;
-  } else {
+  } else if (subpixelAscentDescent &&
+             (-metrics.fAscent < 3 ||
+              -metrics.fAscent + metrics.fDescent < 2)) {
     // For tiny fonts, the rounding of fAscent and fDescent results in equal
     // baseline for different types of text baselines (crbug.com/338908).
     // Please see CanvasRenderingContext2D::getFontBaseline for the heuristic.
-    if (subpixelAscentDescent &&
-        (-metrics.fAscent < 3 || -metrics.fAscent + metrics.fDescent < 2)) {
-      ascent = -metrics.fAscent;
-      descent = metrics.fDescent;
-    } else {
-      ascent = SkScalarRoundToScalar(-metrics.fAscent);
-      descent = SkScalarRoundToScalar(metrics.fDescent);
+    ascent = -metrics.fAscent;
+    descent = metrics.fDescent;
+  } else {
+    ascent = SkScalarRoundToScalar(-metrics.fAscent);
+    descent = SkScalarRoundToScalar(metrics.fDescent);
+
+    int overflowInflationForAscent = ascent < -metrics.fAscent ? 1 : 0;
+    int overflowInflationForDescent = descent < metrics.fDescent ? 1 : 0;
+    if (overflowInflationForDescent) {
+#if OS(LINUX) || OS(ANDROID)
+      // When subpixel positioning is enabled, if the descent is rounded down,
+      // the descent part of the glyph may be truncated when displayed in a
+      // 'overflow: hidden' container.  To avoid that, borrow 1 unit from the
+      // ascent when possible.
+      if (platformData().getFontRenderStyle().useSubpixelPositioning &&
+          ascent >= 1) {
+        ++descent;
+        --ascent;
+        // We should inflate overflow 1 more pixel for ascent instead.
+        overflowInflationForDescent = 0;

[eae, 2017/04/07 17:41:57]

Why? The decent has already been inflated to account for the rounding error.

[Xianzhu, 2017/04/07 18:02:53]

The descent is incremented above (line 158) so it's now bigger than the original
descent. Ascent is decremented so we should inflate overflow for ascent instead.

+        ++overflowInflationForAscent;
+      }
+#endif
     }
-#if OS(LINUX) || OS(ANDROID)
-    // When subpixel positioning is enabled, if the descent is rounded down, the
-    // descent part of the glyph may be truncated when displayed in a 'overflow:
-    // hidden' container.  To avoid that, borrow 1 unit from the ascent when
-    // possible.
-    // FIXME: This can be removed if sub-pixel ascent/descent is supported.
-    if (platformData().getFontRenderStyle().useSubpixelPositioning &&
-        descent < SkScalarToFloat(metrics.fDescent) && ascent >= 1) {
-      ++descent;
-      --ascent;
-    }
-#endif
+    m_fontMetrics.setVisualOverflowInflations(overflowInflationForAscent,
+                                              overflowInflationForDescent);
   }
 
 #if OS(MACOSX)
   // We are preserving this ascent hack to match Safari's ascent adjustment
   // in their SimpleFontDataMac.mm, for details see crbug.com/445830.
   // We need to adjust Times, Helvetica, and Courier to closely match the
   // vertical metrics of their Microsoft counterparts that are the de facto
   // web standard. The AppKit adjustment of 20% is too big and is
   // incorrectly added to line spacing, so we use a 15% adjustment instead
   // and add it to the ascent.
@@ skipping 211 matching lines @@
 float SimpleFontData::platformWidthForGlyph(Glyph glyph) const {
   if (!m_platformData.size())
     return 0;
 
   static_assert(sizeof(glyph) == 2, "Glyph id should not be truncated.");
 
   return SkiaTextMetrics(&m_paint).getSkiaWidthForGlyph(glyph);
 }
 
 }  // namespace blink