This suggestion is a simple and minimal solution to a feature that is missing in Plastic and that we used a lot in TortoiseSVN - the ability to filter a Changeset Diff to the subtree-path that we're viewing the History of.

I have earlier suggested more elaborate changes to support this feature, but the simplest possible fix for this is as follows:

When opening a Changeset Diff from a (2D) History view of a directory (subtree), automatically populate the "Filter:" box (for the Changeset Diff) with the workspace path of the directory that the History view is working against. (This should be done even for "Diff selected changesets".) That way we will see only the items located in the relevant subtree.

(For symmetry/simplicity, this could be done for the History of a single file as well. That is not necessary though, since it would show the same changes as the inline Diff.)

For this to be a flexible solution, there should also be added a checkbox next to the "Filter:" box so we can toggle the filter on/off. (That would be a good feature to have, even on its own...)

The value of this checkbox can be saved to a .conf file. To avoid a change in the default behavior, you can let this checkbox be unchecked initially. Also, when the user manually changes the filter string, you can let this checkbox become checked automatically.

It would be really super to have this simple auto-populated and toggleable filter, since we often want to review the history of changes done in only a sub-tree!

NOTE: In TortoiseSVN this feature is activated by the checkbox "Show only affected paths" in the "SVN Show Log" dialog.