Show previous searches button doesn't enable / disable as expected

It worked under Eclipse Mars during testing.

It doesn't work in Eclipse 2020-03. Sometimes, it gets stuck disabled and won't be enabled. Shutting down Eclipse and opening again seems to fix it; restarting Eclipse doesn't seem to always fix it.

Start by reproducing the previous searches button behavior in Eclipse 2020-03, comparing it with the reported Eclipse Mars behavior. Investigate why the button can remain disabled and whether a full shutdown differs from restarting Eclipse. Done means the button consistently enables and disables according to the availability of previous searches.

Written by the indexing model from the issue text.