Expand explanation for non-coalesced events (#379)

* Expand explanation for non-coalesced events

- a user agents may coalesce events not just if the position is updated, but any of the attributes (as it's not just position changes that can result in an overwhelmingly large number of events)
- tweaked the matching sentence in the `pointerrawupdate` slightly, to use `pointer's attributes` rather than the awkward sounding `pointing device attributes`

Closes #375

* Tweak following sentence of the coalesced events description as well.

* Further expand as this also applies to `pointerrawupdate` events

* Explain which properties usually get coalesced

- includes glossary definition for discrete and coalesced properties

* Exclude discrete keyboard modifier properties from needing to fire pointermove

* Replace the discrete/continuous concept with "measuable properties" instead

due to the fact that `tiltX`, `tiltY`, `twist` being `long` properties would count as discrete under the general definition.

* Remove mention of *key properties in the coalesced events section altogether

no need to mention them, as they don't lead to pointer events firing anyway

* Missing space

* Change "will" to "may"

* Add "continuous" to further clarify the measurable properties concept

index.html

@@ -549,9 +549,8 @@ <h3>The <dfn><code>pointermove</code> event</dfn></h3>
                 <h3>The <dfn><code>pointerrawupdate</code> event</dfn></h3>
                 <p>A user agent MUST <a>fire a pointer event</a>
                 named <code>pointerrawupdate</code> only within a [=secure context=] when
-                a pointing device attribute (i.e. button state, coordinates, pressure, tangential pressure, tilt, twist, or contact geometry) is changed.</p>
-                <p>In contrast with <code>pointermove</code>,
-                user agents SHOULD dispatch <code>pointerrawupdate</code> events as soon as possible
+                a pointer's attributes (i.e. button state, coordinates, pressure, tangential pressure, tilt, twist, or contact geometry) change.</p>
+                <p>In contrast with <code>pointermove</code>, user agents SHOULD dispatch <code>pointerrawupdate</code> events as soon as possible
                 and as frequently as the JavaScript can handle the events.</p>
                 <p>The <code>target</code> of <code>pointerrawupdate</code> events might be different from the <code>pointermove</code> events
                 due to the fact that <code>pointermove</code> events might get delayed or coalesced, and the final position of the event
@@ -942,8 +941,10 @@ <h1><dfn>Coalesced and predicted events</dfn></h1>
             <h2><dfn>Coalesced events</dfn></h2>
 
             <p>For performance reasons, user agents may choose not to send a <code>pointermove</code>
-            event every time the position of a pointer is updated. Instead, they will coalesce
-            (combine/merge) multiple position changes into a single move event. While
+            event every time a <a data-lt="measurable properties">measurable property</a>
+            (such as coordinates, pressure, tangential pressure, tilt, twist, or contact geometry)
+            of a pointer is updated. Instead, they may coalesce (combine/merge) multiple changes into
+            a single <code>pointermove</code> or <code>pointerrawupdate</code> event. While
             this approach helps in reducing the amount of event handling the user agent must perform,
             it will naturally reduce the granularity and fidelity when tracking a pointer position,
             particularly for fast and large movements. Using the
@@ -1403,6 +1404,17 @@ <h1>Glossary</h1>
                 </dd>
             <dt><dfn>hit test</dfn></dt>
                 <dd>The process by which a user agent determines a target element for a pointer event. Typically, this is determined by considering the pointer's location and also the visual layout of elements in a document on screen media.</dd>
+            <dt><dfn>measurable properties</dfn></dt>
+                <dd><p>Measurable properties represent values relating to continuous pointer sensor data that is expressed using a real number or an
+                    integer from a large domain. For pointer events, <code>width</code>, <code>height</code>, <code>pressure</code>,
+                    <code>tangentialPressure</code>, <code>tiltX</code>, <code>tiltY</code>, <code>twist</code>,
+                    <code>altitudeAngle</code>, <code>azimuthAngle</code>, and the [[UI-EVENTS]] Mouse Event model properties
+                    <code>screenX</code>, <code>screenY</code>, <code>clientX</code>, <code>clientY</code> are measurable properties.</p>
+                    <p>In contrast <code>pointerId</code>, <code>pointerType</code>, <code>isPrimary</code>, and the
+                    [[UI-EVENTS]] Mouse Event model properties <code>button</code>, <code>buttons</code>, <code>ctrlKey</code>,
+                    <code>shiftKey</code>, <code>altKey</code>, and <code>metaKey</code> are not considered measurable properties,
+                    as they don't relate to sensor data.</p>
+                    </dd>
             <dt><dfn>pointer</dfn></dt>
                 <dd>A hardware agnostic representation of input devices that can target a specific coordinate (or set of coordinates) on a screen, such as a mouse, pen, or touch contact.</dd>
             <dt><dfn>user agent</dfn></dt>