Pen anti-aliasing is inconsistent on iOS devices #671
Description
Expected Behavior
A pen line that has the same pen size and color and dimensions as another pen line but is translated by an integer number of pixels should have antialiasing that is identical to the other pen line (or close enough to not be noticeable).
Actual Behavior
Vertical lines that are an integer number of pixels apart have differing antialiasing on the two iOS devices I tested. The Windows 10 laptop I tested behaved as expected.
Laptop on Windows 10 2004 with i7-1065G7 processor and integrated graphics on Edge 84.0.522.50:
iPhone XS on iOS 13.5.1 with Apple A12 processor and graphics on mobile Safari:
iPad (6th gen) on iPadOS 13.5.1 with Apple A10 processor and graphics on mobile Safari:
The iOS devices have vertical stripes between pen lines that are more noticeable toward the left and right edges of the stage toward the middle. The iPad has noticeable stripes closer to the center, whereas the iPhone does not.
Steps to Reproduce
Draw vertical pen lines with a pen size of 4 that are 4 pixels apart horizontally.
Here is a project screenshotted above: https://llk.github.io/scratch-gui/develop/#415409213
Operating System and Browser
iOS 13.5.1 Safari
iPadOS 13.5.1 Safari