Created attachment 57331 [details]
Notice the line artifact above the selection rectangle in this screenshot.

The affected Okular version reports itself as 0.11.2, but is a recent SVN checkout of trunk.

The rectangle selection tool leaves behind visual artifacts on pages after a particular sequence of user inputs. In the following rundown, pay special attention to the "slowly" directive in step #3 that seems to be necessary to reproducing the bug:

1. Ctrl+3 to enable the tool.
2. Drag a rectangle right- and upward from the initial mouse cursor position that becomes the selection anchor.
3. Keeping left mouse firmly pressed, *slowly* move the pointer straight downward until the height coordinate of the cursor position is below the height coordinate of the selection anchor by a good distance, i.e. in some sense turning the selection rectangle created in step #2 over vertically.

The result of step #3 is that a visual artifact in the shape of a line spanning the width of the selection rectangle gets left behind just above the rectangle. When releasing the mouse, the selection rectangle disappears, but the artifact is left behind. It also stays in place when moving the document inside the viewport.

The artifact can be wiped from the page by drawing a new selection rectangle that covers it fully and then releasing the mouse.

The attached screenshot shows the artifact above the selection rectangle just before releasing the mouse.