(Pixelmator Pro 1.0.8 on macOS 10.13.3)
While I'd really like fractional line widths for the pen tool, this bug is not about that. It's about what happens when you try to enter a fraction as a line width in an app that doesn't support them.
To reproduce:
Select the pen tool and draw an outline with lines.
With the pen tool still selected remove the fill and set a stroke of 1 px.
- Note the size of the stroke on screen and that the stroke is shown as 1 px in the Tool Options palette.
Now set the stroke to 2 px.
- Note the size of the stroke on screen and that the stroke is shown as 2 px in the Tool Options palette.
Now set the size of the stroke to 1.5 px.
- Note that the size of the stroke on screen matches the 1 px stroke but that the stroke is shown to be 2 px in the Tool Options palette.