Document why whole preview chain is checked for 'next' position

src/terminal.go

@@ -1735,6 +1735,12 @@ func (t *Terminal) separatedByBorder() bool {
 			// A preview window at 'next' position sits right next to the input
 			// section instead. Keep the separator, as the visibility and the
 			// border of the preview window can change at runtime.
+			//
+			// The whole chain is checked because which spec is active depends
+			// on the terminal size, and the resolution (computePreviewSize in
+			// resizeWindows) itself consults noSeparatorLine, so the decision
+			// here cannot depend on its outcome. When a spec other than 'next'
+			// is active, this errs on the side of showing the separator.
 			return false
 		}
 	}