Switching from class to class sometimes is slow

On the memory diff tab, after taking a snapshot, pretend we have classes A and B.

Selecting class A performs computation to calculate the set of retaining paths for to A. Then selecting class B, does the same thing for all the retaining paths of class B. Selecting class A again should be fast, since we have already performed this work. But it appears to take equally as long to compute the set of retaining paths. I suspect we are not caching this computation.

Start in the memory diff tab implementation, following the retaining-path computation triggered when a class is selected. Reproduce the A–B–A selection sequence after taking a snapshot and compare the repeated computation time. Done means returning to class A avoids recomputing its retaining paths while preserving the displayed results.

Written by the indexing model from the issue text.