Hi, Stéph. My understanding of the double click is slightly different. First of all, double click (like search and a few other functions) assues that your first workspace shows the "all tasks" (outline) view. If this is not true, MLO will sometimes open a new workspace with the all tasks view.
When you double click on a task in a workspace other than the first, MLO will switch to the first workspace with the selected task highlighted. I believe that this works well whether the originating view is hierarchical or flat and whether it is filtered or not (in other words, not just to-do lists) This is useful when you want to see where a task sits in your outline and look at its parents, siblings and children. When you double click the task again MLO will switch back to the workspace you started in with the selected task highlighted. This is useful when you are done with your research and are ready to get back to work.
It gets a little dicey when you double click a task in the all-tasks view and you did _not_ use double click to come to all-tasks from some orher workspace. I believe that MLO will make an attempt to find another workspace that includes the selected task and switch there but I would not consider this to be robustly reliable.
-Dwight
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