SUMMARY
clicking on a different file when comparing a directory shows the diff view
at the bottom.
There I scroll down to see the changed line, which is wider than the left or right source view so I scroll horizontally to see the rest of the line.
However the right part of the line is sometimes not shown or shown cut.
For me this looks like a redraw problem since when I scroll by dragging the scrollbar handle, nothing is shown at the part which is moved into the view from the right side. However when I click inside the scrollbar to scroll page-wise to the right, then I can see the right part of the line

SOFTWARE/OS VERSIONS
openSuse Leap 15.0 with
KDE Frameworks 5.59.0
Qt 5.13.0 (built against 5.13.0)
The xcb windowing system