Please note: The following is from the release notes of Notes 8.5.2 so I don't take credit for writing this. Lotus Notes 8.5.2 is in beta and there are no guarantees that the features described here will be in the final product that IBM ships.
You can now set a default double-click action to open widgets in the sidebar, a new window, a float window, or a tab. The default does not impact widgets that have a pre-defined double-click action defined in their XML. The supplied default is newWindow.
Note: See Tech Note 1399534 "Adding a widget custom double-click action" for information about customizing an individual widget's double-click action. (Red: I was unable to find this technote - maybe it hasn't been published yet).
This change enables the control of what action is taken when you double click on a widget in the My Widgets sidebar panel. The default will act on all widgets that do not already have a pre-defined double click action. The new plugin_customization.ini file preference is
com.ibm.rcp.toolbox/doubleClickAction=<option>where valid values for <option> are: