Fix infobar painting issues at fractional scales

chrome/browser/ui/views/infobars/infobar_background.cc

Index: chrome/browser/ui/views/infobars/infobar_background.cc
diff --git a/chrome/browser/ui/views/infobars/infobar_background.cc b/chrome/browser/ui/views/infobars/infobar_background.cc
index 8e732f7bad715db346865243ac35821ecf4e25e9..5e62559bd3b8aa757c3b9f5c5fa5cc70157511ed 100644
--- a/chrome/browser/ui/views/infobars/infobar_background.cc
+++ b/chrome/browser/ui/views/infobars/infobar_background.cc
@@ -73,65 +73,61 @@ void InfoBarBackground::PaintMd(gfx::Canvas* canvas, views::View* view) const {
       infobar->container_delegate();
   SkPath stroke_path, fill_path;
   SkColor separator_color = SK_ColorBLACK;
+  gfx::ScopedCanvas scoped(canvas);
+  // Undo the scale factor so we can stroke with a width of 1px (not 1dp).
+  const float dsf = canvas->UndoDeviceScaleFactor();
+  // Always scale to a whole number to make sure strokes are sharp.
+  // For some fractional DSFs, view size rounding issues may cause there to be a
+  // gap above the infobar (below the horizontal stroke painted by the previous
+  // infobar or the toolbar). We use floor here to "pull" the fill region up a
+  // pixel and bridge the gap. Sometimes this will be 1px too high, so we also
+  // re-draw the horizontal portions of the top separator in case this floor
+  // operation has covered it up. For whole-number DSFs, the floor should have
+  // no effect.
+  auto scale = [dsf](int dimension) { return std::floor(dimension * dsf); };
+  SkScalar arrow_height = scale(infobar->arrow_height());
+  SkScalar infobar_width = scale(infobar->width());
   if (delegate) {
     separator_color = delegate->GetInfoBarSeparatorColor();
     int arrow_x;
     if (delegate->DrawInfoBarArrows(&arrow_x) && infobar->arrow_height() > 0) {
-      SkScalar arrow_fill_height = SkIntToScalar(infobar->arrow_height());
-      SkScalar arrow_fill_half_width =
-          SkIntToScalar(infobar->arrow_half_width());
-      stroke_path.moveTo(SkIntToScalar(arrow_x) - arrow_fill_half_width,
-                         SkIntToScalar(infobar->arrow_height()));
-      stroke_path.rLineTo(arrow_fill_half_width, -arrow_fill_height);
-      stroke_path.rLineTo(arrow_fill_half_width, arrow_fill_height);
+      // Compensate for the fact that a relative movement of n creates a line of
+      // length n + 1 once the path is placed on pixel centers and stroked.
+      SkScalar r_arrow_half_width = scale(infobar->arrow_half_width()) - 1;
+      SkScalar r_arrow_height = arrow_height - 1;
+      SkScalar arrow_tip_x = scale(arrow_x);
+      stroke_path.moveTo(0, r_arrow_height);
+      stroke_path.lineTo(arrow_tip_x - r_arrow_half_width, r_arrow_height);
+      stroke_path.lineTo(arrow_tip_x, 0);
+      stroke_path.lineTo(arrow_tip_x + r_arrow_half_width, r_arrow_height);
+      stroke_path.lineTo(infobar_width - 1, r_arrow_height);
+
+      // Add SK_ScalarHalf to both axes to place the path on pixel centers.
+      stroke_path.offset(SK_ScalarHalf, SK_ScalarHalf);
 
       fill_path = stroke_path;
       fill_path.close();
     }
   }
-  fill_path.addRect(0, SkIntToScalar(infobar->arrow_height()),
-                    SkIntToScalar(infobar->width()),
-                    SkIntToScalar(infobar->height()));
-
-  gfx::ScopedCanvas scoped(canvas);
-  // Undo the scale factor so we can stroke with a width of 1px (not 1dp).
-  const float scale = canvas->UndoDeviceScaleFactor();
-  // View bounds are in dp. Manually scale for px.
-  gfx::SizeF view_size_px = gfx::ScaleSize(gfx::SizeF(view->size()), scale);
-
+  fill_path.addRect(0, arrow_height, infobar_width, scale(infobar->height()));
   SkPaint fill;
   fill.setStyle(SkPaint::kFill_Style);
   fill.setColor(top_color_);
-
-  SkCanvas* canvas_skia = canvas->sk_canvas();
-  // The paths provided by the above calculations are in dp. Manually scale for
-  // px.
-  SkMatrix dsf_transform;
-  dsf_transform.setScale(SkFloatToScalar(scale), SkFloatToScalar(scale));
-  fill_path.transform(dsf_transform);
-  // In order to affect exactly the pixels we want, the fill and stroke paths
-  // need to go through the pixel centers instead of along the pixel
-  // edges/corners. Skia considers integral coordinates to be the edges between
-  // pixels, so offset by 0.5 to get to the centers.
-  fill_path.offset(SK_ScalarHalf, SK_ScalarHalf);
-  canvas_skia->drawPath(fill_path, fill);
+  canvas->DrawPath(fill_path, fill);
 
   SkPaint stroke;
   stroke.setStyle(SkPaint::kStroke_Style);
   const int kSeparatorThicknessPx = 1;
   stroke.setStrokeWidth(SkIntToScalar(kSeparatorThicknessPx));
   stroke.setColor(separator_color);
-  stroke_path.transform(dsf_transform);
-  stroke_path.offset(SK_ScalarHalf, SK_ScalarHalf);
-
-  // The arrow part of the top separator. (The rest was drawn by the toolbar or
-  // the infobar above this one.)
   stroke.setAntiAlias(true);
-  canvas_skia->drawPath(stroke_path, stroke);
+  canvas->DrawPath(stroke_path, stroke);
+
   // Bottom separator.
   stroke.setAntiAlias(false);
+  gfx::SizeF view_size_px = gfx::ScaleSize(gfx::SizeF(view->size()), dsf);
   SkScalar y = SkIntToScalar(view_size_px.height() - kSeparatorThicknessPx) +
                SK_ScalarHalf;
-  SkScalar w = SkIntToScalar(view_size_px.width());
-  canvas_skia->drawLine(0, y, w, y, stroke);
+  SkScalar w = SkFloatToScalar(view_size_px.width());
+  canvas->sk_canvas()->drawLine(0, y, w, y, stroke);
 }