stray background text not painted over in the context window when scrolling with Ctrl+D

[wmusial]

steps to reproduce:

open a file with a long line somewhere at the beginning, and nested contexts within half-window later. In my example the file begins with:

//
// This program is proprietary and confidential MySoftwareCompany software
(13 more lines)
namespace foo_bar {

    //***************************************************************************************************
    class FooBarBaz
        : public ABC,
          public ExecConnection
    {
    public:

pressing Ctrl+D to scroll down one page results in the following context window:

namespace foo_bar {
    class FooBarBaz ··· {                                             are
       -------------------------------------------------------------------------------------- <context.vim>

observe the dangling "are", a substring of the comment line, that hasn't been painted over in the context window.

This only happens when jumping with Ctrl+D. scrolling down one line at a time works fine.