If your widgets no longer show up properly in Connections 5.5...

This happened to us as we are validating ProjExec with Connections 5.5. As a project is related to a community, we have community based widgets that render ProjExec data within the community. But, as of 5.5, our widgets did not display anymore in the middle column:
But it works when being added to the right column. Damned, we didn't change anything, so it must be something introduced in C5.5. We looked at the existing IBM widgets definition and saw that they had a an attribute called "themes" that might be related:
<widgetDef defId="ImportantBookmarks" description="importantBookmarksDescription" modes="view" themes="wpthemeNarrow wpthemeWide wpthemeThin wpthemeBanner" uniqueInstance="true" primaryWidget="false"  

We tried that on our own widgets and they now work as expected! This seems to be a new "feature" as it is not (yet) documented (https://www-01.ibm.com/support/knowledgecenter/SSYGQH_5.5.0/admin/admin/t_admin_communities_develop_custom_widgets.dita?lang=en). On the other hand, the IBM portal documentation describes it (https://www-01.ibm.com/support/knowledgecenter/SSYJ99_8.5.0/dev-theme/themeopt_cust_layouttemp.html).

I submitted a PMR to at least get the documentation updated and, best scenario, maintain the backward compatibility when this attribute is not present. I'm sure that our customers migrating to 5.5 will come to our support team with this issue.

Update: Mikkel provided a detailled post on the subject: http://lekkimworld.com/2016/01/26/custom_widgets_for_ibm_connections_v_5_5_and_what_the_themes_attribute_mean.html