The most useful thing in this thread is buried ten replies back and it is poorboy2001's own finding. With both the Windows autohide and the Start11 autohide turned off, the line stopped for about ten days. joeshades, that is worth trying before anything else, because it costs nothing and it is the only change in here that has ever moved the symptom.
Autohide showing up in a taskbar drawing problem is not a coincidence on our side either. The icon size, button width and height options carry the documented caveat that they only apply properly when you are not using OS autohide, and 1.43 added warnings that taskbar size changes need the enhanced taskbar and likewise for autohide. It is a special case in that path rather than an ordinary setting.
If autohide does turn out to be involved and you want it back, it is not all or nothing. 2.55 added a mode that only hides the taskbar when a window is actually over it, labelled since 2.72 as "Only hide when a window is maximized or overlapping the taskbar". That is the compromise worth testing rather than living without autohide.
One thing worth saying plainly about the driver theory from June. Two people are reporting it today, so a newer NVIDIA driver is not a general fix for this and nobody should stop looking there. The fact that Restart Explorer clears the line also fits a redraw problem more comfortably than a driver one.
poorboy2001, the detail in your earlier post about it turning up when the laptop wakes and windows reposition across displays is the part I would keep in front of anyone who picks this up. A display configuration change in the middle is a much more specific thing to chase than an intermittent artifact.
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