The layout dialect doesn't do that right now, so you need to specify the full title. (Oh, and you don't need to specify layout:fragment on the <title> element - the layout dialect automatically overrides the parent template's <title> :) )
I can imagine such a feature would be useful (I've got breadcrumbs in the title on my website by spelling out the website name in every template. Something like this would save me having to do that). I've had a quick think about it and have some ideas (not sure if they're good ideas though).
One idea is to specify a setting in the constructor/setter to set that behaviour globally:
// Make templates append to the right of the layout's title:
// Same thing, but in Spring XML configuration:
<bean id="templateEngine" class="org.thymeleaf.spring3.SpringTemplateEngine">
<property name="templateResolver" ref="templateResolver"/>
<property name="extendTitle" value="right"/>
Maybe another way would be to create a new attribute that you set on <title>, making it only apply to certain pages:
Or maybe a combination of both to give developers greater control.