It is definitely a screen artefact. It does not occur systematically. The scroll position of the document within the screen (not only the window) must be such that the rendering engine (or the window manager) meets an ambiguity as to where to draw the line. Computation gives it between two pixels rows. In some circumstances with thin lines, this may cause the line to be draw twice on two adjacent screen rows.
Change the zoom level or scroll (this latter quite hard because scroll must be offset a non integral number of pixels) and the line aspect changes.
This occurs only with thickness smaller than one pixel, which is the case with 0.1pt (screen density varies from 72dpi to ~100dpi, i.e. a pixel is between 1pt and 0.7pt. Therefore 0.5pt and 0.1pt are clearly sub-pixel dimensions, though you will less often notice the artefact at 0.5pt.
Since printers have a resolution of 300 or 600dpi for consumer-grade models, a pixel is 0.24 or 0.12pt wide. The artefact will rarely happen, though it is possible, but you will hardly notice it.
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