When you drag the brush size slider, the icon on the toolbar renders over itself as it animates, making it illegible. You can see the issue in the attached video captured on iPhone.
Also- the icon is of the brush stamp and scales down to nothing when the size goes to 1- I find this pretty impractical. I often work with small brushes and it effectively makes the brush icon invisible most of the time. There also is a bug that makes the little radius number below the icon not render sometimes which makes things worse.
I think a very simple improvement would be to just not animate the size of the brush stamp on the icon. Just leave it a constant scale that is large enough to see. Let the radius number inform you of the current size. The visual scale of the icon can’t be a great preview of the brush size anyway because it’s limited to the size of the toolbar height.
There are some brushes with very transparent stamps that are hard to see on the icon as well, even when they are larger… I’m not sure about what to do to improve this other than trying normalizing the opacity range of the stamp for the toolbar icon. But that may look strange in some cases.
Thanks for your consideration!
