🐛 Use border-box to prevent losing subpixel AA

When highlights are positioned outside the line box, they could get
hidden by a subsequent tile (because of its opaque background). As a
result, subpixel anti-aliasing gets disabled by Chrome.

Using `border-box` as the default `boxSizing` allows highlights to
always fit within the line (except if a styling like `margin` is
applied).

src/highlights-component.coffee

@@ -76,6 +76,10 @@ class HighlightsComponent
       unless oldHighlightState.regions[i]?
         oldHighlightState.regions[i] = {}
         regionNode = document.createElement('div')
+        # This prevents highlights at the tiles boundaries to be hidden by the
+        # subsequent tile. When this happens, subpixel anti-aliasing gets
+        # disabled.
+        regionNode.style.boxSizing = "border-box"
         regionNode.classList.add('region')
         regionNode.classList.add(newHighlightState.deprecatedRegionClass) if newHighlightState.deprecatedRegionClass?
         @regionNodesByHighlightId[id][i] = regionNode