Indeed!
Blending colors is surprisingly hard.
I've pushed an update that addresses most of your concerns.
Not perfect yet, but an improvement.
Dev version with these changes on https://www.stef.be/dpaint/dev/
Also in both cases the brush area remained intact (it was just 'moving around') instead of being smudged, it is more obvious in the Firefox shot because i only did a simple circular motion but can also be seen in the Falkon shot in that there is a floating corner at the top right corner of the orange box that was 'dragged' from the left side (i was doing a horizontal motion to show that the pixels above the brush were affected even though there wasn't any vertical motion to push the orange colors up to the blue area).