Thanks for looking at this!
Rather than managing the surface yourself (and in particular, making a copy of it on each mouse move), I'd suggest making use of the ToolLayer, which is a scratch layer that's always available for tools to make use of, and is displayed right above the current layer when it's not hidden.
A good example of this is the freeform shape tool which draws to the tool layer while moving the mouse, and then just does a final draw to the actual layer on mouse up: https://github.com/PintaProject/Pinta/blob/master/Pinta.Tools/Tools/FreeformShapeTool.cs#L85

@cameronwhite Thanks! I'll be looking into that soon

Thanks, the ToolLayer approach looks better 👍

Some notes from doing a bit more testing:

• Doing a single click (not a click + drag) doesn't cause anything to be drawn now
• Using a brush width of 1 seems to sometimes produce some odd results, like drawing a filled polygon from the whole sequence of mouse movements. Might be some odd interaction with the DrawSinglePixelLine() code path?
• If possible, it would be great to move more of the logic into the PlainBrush to avoid special casing things too much. If necessary there can be more virtual functions added to BasePaintBrush to facilitate this, e.g. requesting that the brush should use the tool layer

Hi @cameronwhite, I just pushed a few commits that deals with the issues above:

1. Not clearing the ToolLayer in OnMouseDown() seems to have fixed the single click interaction (which, to be frank, I don't know how both the issue and the solution works).

2. For the brush width of 1, I was able to bypass checking and calling DrawSinglePixelLine() completely because the code will always force out a line, which looks like this:

3. path seemed to be the only variable that could be brought to the inner PlainBrush class so I move it over. Currently the only code that depends on the type of active_brush is this one.

3. I was able to bring the path and surf variable into PlainBrush and clear them inside PlainBrush, which should clean up code and eliminate some conditionals.

1. Not clearing the ToolLayer in OnMouseDown() seems to have fixed the single click interaction (which, to be frank, I don't know how both the issue and the solution works).

I think this was because the base class OnMouseDown() calls the OnMouseMove handler to avoid dealing with single-click as a special case (

2. For the brush width of 1, I was able to bypass checking and calling DrawSinglePixelLine() completely because the code will always force out a line, which looks like this:

I think the behaviour looks okay when dragging to draw a line, but doing just a single click produces some odd asymmetric antialiasing which you can see in the screenshot. The old behaviour produced a "plus" shape which I think would be more expected for that scenario

Otherwise, the changes are looking good!

Thanks for telling me, @cameronwhite
I'll try to tackle the remaining problems in a few days

Looks good, thanks for contributing!