en the <code>mask</code> is
assigned to a second display object, it is removed as the mask of the
first object, and that object's <code>mask</code> property becomes
<code>null</code>.</p></div></div><div class="field "><a name="mouseX"></a><h3 class="anchor"><code><span class="label">read only</span><a href="#mouseX"><span class="identifier">mouseX</span></a>:<a class="type" title="Float - The standard Float type, this is a double-precision IEEE 64bit float." href="../../Float.html">Float</a></code></h3><di