1. Create a new image (1024x1024 for example)
2. Zoom in significantly (500% for example)
3. Select a brush
4. Click on a point of the left of the canvas
5. Without changing the zoom level, move to the right of the canvas
6. While holding shift, click on a point of the right of the canvas.
7. Zoom out
You see a straight line starting somewhere in the right-half of the canvas, and finishing where you clicked on step 6. Notice how the line doesn't connect the first click point (step 4) with the second one (step 6), but it has the right direction.
Pixelmator should have created a straight line from the first point to the second, without any gaps in between.
macOS High Sierra 10.13.6
MBP 15" 2018 - 2.6Ghz i7 - 32GB Ram - Radeon Pro 560X
Pixelmator 1.1.3 Monsoon
Is something not working like it should? Let us know.