Weird blue outlines with Lock Transparency

[markw]

I think, given the relatively small size of your canvas, it might be better to come at this problem from another direction perhaps
Instead of “locking out” transparent pixels, maybe “locking in” as it were, the painted ones might give you better results.
Two options would be:
1: Select Layer Content = This will bring up the “marching ants” around the content of any given layer and nothing outside this selected area can be worked on.
The animated doted line can sometimes be a bit distracting while painting, and can be a bit more work for weaker CPUs on large canvases to track if zooming and panning the canvas a lot but on a small canvas I doubt it would be an issue.
2: New Stencil From Layer Content = This works much the same as above, but creates a protective red stencil around the immediate working area. Outside of the stencil you would still be able to paint on the layer.
Both these options can be accessed by right clicking on the desired layer in the Layers Panel and choosing them from the popup menu.
Or from the Menu bar: Tools > Layer Options > ...
If these two options are useful, you can of course set key shortcuts to activate them more quickly in AR’s Keyboard Shortcuts Preferences.