Pointer Offset in iPad Safari

When I load dev.graphite.rs on my iPad Safari, start a new canvas and then select the paintbrush tool, drawing is broken. That is, wherever I drag my finger, the drawing will be done about 1/4 of the screen up and 1/4 of the screen left of where I drag my finger.

Reproduce the issue on an iPad using Safari at dev.graphite.rs: create a new canvas, select the paintbrush, and drag a finger. Trace the canvas pointer or touch-position handling, then verify that the brush follows the contact point rather than appearing offset up and left. Done means drawing aligns with the finger on iPad Safari without regressing other canvas input.

Written by the indexing model from the issue text.