[scrollable shelf] Fix the issue of one-pixel gap

There may be a one-pixel gap between the gradient zone and the arrow
button (not always). It is caused by the rounding error during painting.
The easiest way to fix this is to enlarge the gradient zone by one
pixel. It does not introduce other visual side effects.

Bug: 1018965
Change-Id: Ifceb1310adee0e907ec3fc1ed7e88ea50af2b7fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885092
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710377}

ash/shelf/scrollable_shelf_view.cc

@@ -1203,20 +1203,26 @@ ScrollableShelfView::FadeZone ScrollableShelfView::CalculateStartGradientZone()
     return FadeZone();
 
   if (is_horizontal_alignment) {
-    int x;
+    int gradient_start;
+    int gradient_end;
 
-    // Calculates the start location on x-axis of the gradient zone.
+    // Calculates the bounds of the gradient zone. Enlarge the gradient zone by
+    // one-pixel to offset the potential rounding error during rendering (we
+    // also do it in CalculateEndGradientZone()).
     if (ShouldAdaptToRTL()) {
       const gfx::Rect mirrored_left_arrow_bounds =
           GetMirroredRect(left_arrow_bounds);
-      x = mirrored_left_arrow_bounds.x() - kGradientZoneLength;
+      gradient_start = mirrored_left_arrow_bounds.x() - kGradientZoneLength;
+      gradient_end = mirrored_left_arrow_bounds.x() + 1;
     } else {
-      x = left_arrow_bounds.right();
+      gradient_start = left_arrow_bounds.right() - 1;
+      gradient_end = left_arrow_bounds.right() + kGradientZoneLength;
     }
-    zone_rect = gfx::Rect(x, 0, kGradientZoneLength, height());
-  } else {
     zone_rect =
-        gfx::Rect(0, left_arrow_bounds.bottom(), width(), kGradientZoneLength);
+        gfx::Rect(gradient_start, 0, gradient_end - gradient_start, height());
+  } else {
+    zone_rect = gfx::Rect(0, left_arrow_bounds.bottom() - 1, width(),
+                          kGradientZoneLength + 1);
   }
 
   fade_in = !ShouldAdaptToRTL();
@@ -1235,20 +1241,23 @@ ScrollableShelfView::FadeZone ScrollableShelfView::CalculateEndGradientZone()
     return FadeZone();
 
   if (is_horizontal_alignment) {
-    int x;
+    int gradient_start;
+    int gradient_end;
 
-    // Calculates the start location on x-axis of the gradient zone.
     if (ShouldAdaptToRTL()) {
       const gfx::Rect mirrored_right_arrow_bounds =
           GetMirroredRect(right_arrow_bounds);
-      x = mirrored_right_arrow_bounds.right();
+      gradient_start = mirrored_right_arrow_bounds.right() - 1;
+      gradient_end = mirrored_right_arrow_bounds.right() + kGradientZoneLength;
     } else {
-      x = right_arrow_bounds.x() - kGradientZoneLength;
+      gradient_start = right_arrow_bounds.x() - kGradientZoneLength;
+      gradient_end = right_arrow_bounds.x() + 1;
     }
-    zone_rect = gfx::Rect(x, 0, kGradientZoneLength, height());
+    zone_rect =
+        gfx::Rect(gradient_start, 0, gradient_end - gradient_start, height());
   } else {
     zone_rect = gfx::Rect(0, right_arrow_bounds.y() - kGradientZoneLength,
-                          width(), kGradientZoneLength);
+                          width(), kGradientZoneLength + 1);
   }
 
   fade_in = ShouldAdaptToRTL();