If you paint a text over the QToolBar's background with the enabled unified title and tool bar look on macOS, text glyphs appear too blurry, as if it was too much anti-aliasing applied (or as if the same text was rendered several times at the same location so that final transparency of anti-aliased pixels decreased).

I'm attaching a test program and a screenshot that demonstrates the problem. The test program adds two QLabel instances to the tool bar where the first one has no (transparent) background and the second one fills it with a solid color. The text on the second one looks right. There are also two tool buttons demonstrating the same effect.

Note that disabling the unified title and tool bar look makes it work right too, regardless of whether the label's background is transparent or not.