gather
For every existing fiber that isn't already captured by another debugger, bind that fiber to this debugger. Those fibers are not permitted to run unless this debugger says they may. Any fibers launched after this point (say, to compute a print representation or evaluate an expression) will not be captured by this debugger.
Note that this operation will block the current thread (which should be a UI-spawned thread) while holding the runtime at a safe point, to ensure no other fibers are running, and to ensure other debuggers don't conflict with this one.
Parameters
then
An action to execute (in a different Thread) after the fibers have been gathered.