An advisory note on the use of backgrounds on regions in combination with forcedDisplay

[plehegar]

This issue is created to fulfil Action-314.

Rationale:

Since forcedDisplay affects the computed value of the tts:visibility property it has no effect on whether or not the hidden/visible content is temporally active. This means that if non-forced content is assigned to a region with a background colour then the background will be shown whenever the content is active even if it is hidden. This is likely to be unexpected behaviour for some readers of the specification, who may imagine that by using a setting of tts:showBackground of "whenActive" they can prevent this effect.

(incidentally they'd be correct in thinking this if forcedDisplay were changed to do what its name suggests and affect tts:display, which arguably would be more useful functionality)

Proposal:

Include a non-normative note such as the following:
<--
NOTE

If the forcedDisplay attribute is used for content in combination with regions that have a non-transparent computed background color then authors should be aware that those regions' backgrounds will be drawn whenever the selected content is active, even if the computed tts:visibility of that content is "none". One strategy for avoiding this scenario would be to assign content elements only to regions that have the same value of forcedDisplay.
-->

(raised by Nigel Megitt on 2014-08-01)
From tracker issue http://www.w3.org/AudioVideo/TT/tracker/issues/331