Just installed the latest release (was on 3.4.18.0), and ran into a small, (but annoying) behavior change:

If I click on a pending file, SourceTree shows the file's diff in the diff pane. If I switch to a different app (usually a browser), then switch back to SourceTree, the diff will show in the diff pane for a moment, then disappear; it's replaced by the "Select a file to view the diff" message. The only way to get the diff back is to click on a different file, then back to the file I was viewing.

Not sure if it matters, but I do have an external diff viewer configured. Of course, even with the external viewer, the built-in diff pane should keep the diff showing when switching from/back to SourceTree.