The TransactionOverviewWidget class was added in bitcoin-core/gui#176 as a header-only one.
Apparently, in upcoming CMake project, CMake AUTOMOC could be integrated better/simpler, if QObject-derived class implementation been placed into a source file.
From our Developer Notes:
Implementation code should go into the
.cppfile and not the.h, unless necessary due to template usage or when performance due to inlining is critical.