Add lowLatency flag to CanvasRenderingContext2DSettings.

Developers of stylus-based drawing applications (e.g. Google Keep)
have found this hint to be critical in order to be competitive with
native applications. It has been prototyped in Chromium and yields
significant latency improvements there.

Partial fix for whatwg#4087.

source

@@ -59862,6 +59862,7 @@ enum <dfn>CanvasFillRule</dfn> { "<span data-x="dom-context-2d-fillRule-nonzero"
 
 dictionary <dfn>CanvasRenderingContext2DSettings</dfn> {
   boolean <span data-x="dom-CanvasRenderingContext2DSettings-alpha">alpha</span> = true;
+  boolean <span data-x="dom-CanvasRenderingContext2DSettings-lowLatency">lowLatency</span> = false;
 };
 
 enum <dfn>ImageSmoothingQuality</dfn> { "<span data-x="dom-context-2d-imageSmoothingQuality-low">low</span>", "<span data-x="dom-context-2d-imageSmoothingQuality-medium">medium</span>", "<span data-x="dom-context-2d-imageSmoothingQuality-high">high</span>" };
@@ -60123,6 +60124,12 @@ interface <dfn>Path2D</dfn> {
     <p>If the <code data-x="dom-CanvasRenderingContext2DSettings-alpha">alpha</code> setting is
     provided and set to false, then the canvas is forced to always be opaque.</p>
 
+    <p>If the <code data-x="dom-CanvasRenderingContext2DSettings-lowLatency">lowLatency</code>
+    setting is provided and set to true, then the user agent will attempt to optimize the rendering
+    of the canvas to reduce latency, including potentially rendering from the front buffer. This
+    might desynchronize the canvas's display from the rest of the page and induce tearing on some
+    displays.</p>
+
    </dd>
 
    <dt><var>context</var> . <code subdfn data-x="dom-context-2d-canvas">canvas</code></dt>
@@ -60166,6 +60173,20 @@ interface <dfn>Path2D</dfn> {
   created <span>output bitmap</span> with its <span data-x="concept-canvas-alpha">alpha</span>
   flag set to false will result in a fully-opaque gray square.</p>
 
+  <p>The <code>CanvasRenderingContext2D</code> object also has a <dfn
+  data-x="concept-canvas-lowLatency">lowLatency</dfn> flag, which can be set to true or false.
+  Initially, when the context is created, its <span
+  data-x="concept-canvas-lowLatency">lowLatency</span> flag must be set to false. When a
+  <code>CanvasRenderingContext2D</code> object has its <span
+  data-x="concept-canvas-lowLatency">lowLatency</span> flag set to true, then the user agent will
+  attempt to optimize the rendering of the canvas to reduce latency, including potentially rendering
+  from the front buffer. This might desynchronize the canvas's display from the rest of the page and
+  induce tearing on some displays.</p>
+
+  <p class="note">The <code data-x="concept-canvas-lowLatency">lowLatency</code> setting is useful
+  and necessary when implementing certain kinds of applications, such as drawing applications, where
+  the delay between input and rasterization is critical.</p>
+
   <hr>
 
   <p>The <code>CanvasRenderingContext2D</code> 2D rendering context represents a flat linear
@@ -60227,6 +60248,11 @@ interface <dfn>Path2D</dfn> {
      <dt><dfn><code data-x="dom-CanvasRenderingContext2DSettings-alpha">alpha</code></dfn></dt>
      <dd>If false, then set <var>context</var>'s <span data-x="concept-canvas-alpha">alpha</span>
      flag to false.</dd>
+
+     <dt><dfn><code
+     data-x="dom-CanvasRenderingContext2DSettings-lowLatency">lowLatency</code></dfn></dt>
+     <dd>If true, then set <var>context</var>'s <span
+     data-x="concept-canvas-lowLatency">lowLatency</span> flag to true.</dd>
     </dl>
    </li>