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/** \page dw-stacking-context Handling stacking contexts
Stacking Context and dw::core::StackingContextMgr
=================================================
For the definition of stacking contexts, see CSS 2.1 specification,
- <a href="http://www.w3.org/TR/CSS2/visuren.html#z-index">section
9.9.1: Specifying the stack level: the 'z-index' property</a> and
- <a href="http://www.w3.org/TR/CSS2/zindex.html">appendix E</a>.
A widget establishes a stacking context when it is positioned and its
style value of *z-index* is different from *auto* (see
dw::core::StackingContextMgr::isEstablishingStackingContext). In this
case, it is assigned an instance of dw::core::StackingContextMgr,
which has also access to the widgets establishing the child contexts.
Stacking Order
==============
The stacking order influences
1. the order in which child widgets are drawn (dw::core::Widget::draw),
and
2. the order in which mouse events are dispatched to child widgets
(dw::core::Widget::getWidgetAtPoint).
The first is done from bottom to top, the latter from top to bottom.
I'm here referring to the simplified description in
<a href="http://www.w3.org/TR/CSS2/visuren.html#z-index">section
9.9.1</a>. The table shows a recommended order for the implementations
of dw::core::Widget::draw and dw::core::Widget::getWidgetAtPoint
(for the latter, read from bottom to top):
<table>
<tr>
<th> CSS specification <th> Drawing <th> Mouse events
<tr>
<td> *1. the background and borders of the element forming the
stacking context.*
<td> dw::core::Widget::drawBox
<td> Nothing necessary.
<tr>
<td> *2. the child stacking contexts with negative stack levels (most
negative first).*
<td> dw::core::StackingContextMgr::drawBottom (when defined)
<td> dw::core::StackingContextMgr::getBottomWidgetAtPoint (when defined)
<tr>
<td> *3. the in-flow, non-inline-level, non-positioned descendants.*
<td rowspan="4"> When (i) widget specific content is drawn, then (ii)
dw::oof::OOFAwareWidget::drawOOF is called, this will
have this effect:
1. all in-flow elements are drawn,
2. floats are drawn and
3. positioned elements with *z-index: auto* are drawn
(latter two done by
dw::oof::OOFAwareWidget::drawOOF, in this order).
This order differs from the specified order, but
since floats and in-flow elements do not overlap,
this difference has no effect.
Drawing in-line elements, floats and positioned
elements with *z-index: auto* and should avoid
duplicate calls: Widgets drawn by
dw::core::StackingContextMgr::drawBottom and by
dw::core::StackingContextMgr::drawTop should be
excluded here. This can be tested with
dw::core::StackingContextMgr::handledByStackingContextMgr.
<td rowspan="4"> Likewise, the implementation should (i) test
dw::oof::OOFAwareWidget::getWidgetOOFAtPoint, and
(ii) search through the chilren. Also, duplicate
calls should be avoided using
dw::core::StackingContextMgr::handledByStackingContextMgr.
There are already the implementations
dw::core::Widget::getWidgetAtPoint (ignoring
dw::oof::OutOfFlowMgr) and
dw::oof::OOFAwareWidget::getWidgetAtPoint (including
dw::oof::OutOfFlowMgr).
<tr>
<td> *4. the non-positioned floats.*
<tr>
<td> *5. the in-flow, inline-level, non-positioned descendants,
including inline tables and inline blocks.*
<tr>
<td> (What about positioned elements with *z-index: auto*? Seems to be
missing in
<a href="http://www.w3.org/TR/CSS2/visuren.html#z-index">section
9.9.1</a>, but mentioned in
<a href="http://www.w3.org/TR/CSS2/zindex.html">appendix E</a>,
item 8.
<tr>
<td> *6. the child stacking contexts with stack level 0 and the
positioned descendants with stack level 0.*
<td rowspan="2"> dw::core::StackingContextMgr::drawTop (when defined)
<td rowspan="2"> dw::core::StackingContextMgr::getTopWidgetAtPoint
(when defined)
<tr>
<td> *7. the child stacking contexts with positive stack levels (least
positive first).*
</table>
Note: This is not quite in conformance with the specification: this
description refers to any widget, not only widgets establishing a
stacking context. Does this make a difference?
*/
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