The brush tool is super-fast now! I changed it from drawing non-filled circles until a the complete circle was filled to determining if a point is in a circle then drawing. Unfortunately the UI on the Tools page and the File page got completely borked, I'm looking into what caused that. Next update will add a zoom ability to canvases.
Alpha v0.2 is ready! Introduced some bug fixes and added zoom and scroll features, and did a little redesigning. Now it's actually kind of useful. See the Changelog for more info.
Partial fixes- the area now updates in the correct spot, but there are still some graphical issues, and crashes with large y offsets are still common.
Fixed the crashing, I forget my own method returned nullptr in cases where the pixel lies outside the Layer. Oops.