.Screenshot A (LOWER) is a "clean" SDRC window.
Screenshot B (UPPER) shows an extra "border" just below the title bar (near the top of the menu tabs) and continued down the left hand side until the top of the DSP window.
This "extra" can be induced by a mouse-over to any of the Low, High and Zoom sliders. And also by opening windows such as Tools>Options, Help> About.
It is most clearly recognised as a horizontal line across the middle of the top-left circular system menu.
Various actions clear these borders: clicking on any menu (tab) or a menu item; mouse-over the system menu; maximise/restore.
However, switching applications and back makes those borders reappear.
Finally I came to realise that switching applications is all it takes to produce the effect.
This is a fresh install of 3.0.26 but I've no idea when the behaviour may have been introduced, being away from SDR for quite a while.
The big difference now is I'm having to use a slow i5 laptop with only 6 GB RAM.