Add client coordinate definition and note on usage during scroll (#85)

There are ReSpec errors because "client coordinate system" is used but not defined. I didn't find a good definition from another spec, so I've added a definition here.

Per #39, add a note on the fact that authors may want to adjust coordinates during scroll if they want to the IME window position to move with the content.

Closes #39.

index.html

@@ -113,8 +113,8 @@ <h4>EditContext state</h4>
  <li><dfn>composition start</dfn> which refers to the offset in [=text=] representing the start position of the text being actively composed. The initial value is 0.</p></li>
  <li><dfn>composition end</dfn> which refers to the offset in [=text=] representing the end position of the text being actively composed. The initial value is 0. [=composition end=] must always be greater than or equal to [=composition start=].</li>
  <li><dfn>text formats</dfn> which is an array of [=text format=]. The array is initially empty.</li>
- <li><dfn>control bounds</dfn> is a rectangle describing the area of the screen in which [=text=] is displayed. It is in the [=client coordinate=] system and the initial x, y, width, and height all 0.</li>
- <li><dfn>selection bounds</dfn> is the rectangle describing the position of selection. It is in the [=client coordinate=] system and the initial x, y, width, and height are all 0.</li>
+ <li><dfn>control bounds</dfn> is a {{DOMRect}} describing the area of the viewport in which [=text=] is displayed. It is in the [=client coordinate system=] and the initial x, y, width, and height are all 0.</li>
+ <li><dfn>selection bounds</dfn> is a {{DOMRect}} describing the position of selection. It is in the [=client coordinate system=] and the initial x, y, width, and height are all 0.</li>
  <li><dfn>codepoint rects start index</dfn> which is an offset into [=text=] that respresents the position before the first codepoint whose location is reported by the first member of [=codepoint rects=] array.</li>
  <li><dfn>codepoint rects</dfn> is an array of {{DOMRect}} defining the bounding box of each codepoint. The array is initially empty.</li>
  </ul>
@@ -135,6 +135,36 @@ <h4>EditContext state</h4>
  from the [=Text Input Service=]. The user agent will indicate which positions are
  required by firing {{CharacterBoundsUpdateEvent}}.
  </p>
+ <p>
+ [=Control bounds=], [=selection bounds=], and [=codepoint rects=] are given in the
+ <dfn>client coordinate system</dfn>, which is defined as a two-dimensional Cartesian
+ coordinate system (x, y) where the origin is the top-left corner of the
+ <a href="https://drafts.csswg.org/cssom-view/#layout-viewport">layout viewport</a>,
+ the x-axis points towards the top-right of the
+ <a href="https://drafts.csswg.org/cssom-view/#layout-viewport">layout viewport</a>,
+ and the y-axis points towards the bottom-left of the
+ <a href="https://drafts.csswg.org/cssom-view/#layout-viewport">layout viewport</a>.
+ The units of the client coordinate system are
+ <a href="https://drafts.csswg.org/css-values/#px">CSS pixels</a>.
+ </p>
+ <div class="note">
+ <p>
+ Since EditContext bounds are defined in
+ <a data-lt="client coordinate system">client coordinates</a>, the coordinates
+ indicating a given piece of content on a page will change as the user scrolls the
+ document even if the content itself does not change position in the document. A
+ scenario where authors may want to take this into account is the case where the user
+ scrolls the page where the user has an active composition. If the author does not
+ update the EditContext's bounds information (e.g. during a scroll event listener),
+ the IME window may no longer line up with the text being composed for the duration
+ of the composition.
+ </p>
+ <p>
+ However, some platforms do not adjust IME windows during an active composition,
+ so updating bounds information mid-composition does not guarantee that the IME
+ window will be repositioned until it's closed and reopened.
+ </p>
+ </div>
  <h4>Association and activation</h4>
  <p>
  An {{EditContext}} has an <dfn data-for="edit-context">associated element</dfn>, an {{HTMLElement}}.