The left-hand side of one of my open Safari windows is filled with an intrusion, (as shown in the image I'll try and insert below). Switching between tabs in the window makes no difference: the intruder remains in place. I can only scroll left and right in the part of the window not occupied by the intruder. However, other open Safari windows are not affected. Does anyone have an idea what has happened? Is it a malfunction of Safari? Is it likely to be something to do with malware?