Looks minimalistic and nice to use. I might try making a pixel icon for my app with it.
The name of this project brought many memories of the HP 48G to the forefront of my mind, after so many years. Its 1 bpp pixelated icons and drawings were indeed called GROBs - there are collections of these online. What a coincidental but fine name for a pixel-capable graphics editor.
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Great effort and reason to create this! I'm accustomed to using the Magic Wand tool in Paint.net[1] and Pinta[2] to select pixels based on color. Any chance you could add that functionality?
[1] https://www.getpaint.net/doc/latest/MagicWand.html
[2] https://www.pinta-project.com/user-guide/wand/