That's interesting because your example shows that pixel rounding is affecting the objects movement at layout coordinates, and not the CSS canvas coordinates, which is in contrast to how it is described in the manual.
The viewport is 9x9.
With pixel rounding on the object only moves 1px in layout coordinates when it approaches 1px.
With pixel rounding off the object can move to fractional positions on the layout.
However, the CSS canvas size is much larger, depending on the scale, and the effects of pixel rounding are unclear in this example.
I did realise what was happening though that was confusing for me before.
In this modified example, scaling is turned off, the canvas size is set to 200x200 and there is a single red pixel. When moving this across the screen it does only move 1 px at a time, regardless of pixel rounding being on or off.
The issue is with Windows DPI scaling. If the same example is run with 125% scaling on, every 4 layout pixels moved, the single pixel is stretched across 2 css/device pixels even if pixel rounding is on.
It's unfortunate that pixel rounding is not able to factor in DPI scaling as it makes the feature a bit redundant on a lot of devices. It seems that pixel rounding is a pre-scaling operation, which explains why it does not appear to work when the layout is not perfectly scaled either by Construct or Windows.
EDIT: It's clear that Windows scaling implementation is the issue. If anyone knows a way to force disable scaling in Chromium I'd be keen to know.