Does anyone know what these strange red blocks mean on my scroll bar (in VS2019) ?

They aren't related to the source lines - i.e. if I scroll the text window, the blocks remain in the same positions; they don't move as I scroll. And I only ever see them if I load one particular source file - even if it's unedited and even if I close that file and re-open it...

Name:  Capture-19.jpg
Views: 102
Size:  26.2 KB