If you’re a stickler for pixel-level details, you may notice that, under Light, Medium and Strong font smoothing setting in OS X, light-colored text set on dark-colored backgrounds will look thicker than dark text on light background.

On the contrary, if you set your font smoothing to “Standard – best for CRT”, you’ll notice that both text settings look optically even.

(You can try out these settings by going to System Preferences → Appearances → Font smoothing style.)

So under Medium font smoothing, highlighted text on Mail.app mailbox menu should look thicker than normal text, like this:

But when Mail.app text rendering bug kicks in, the thickening no longer occurs. Note how both texts now look optically even.

It seems that, for a brief moment, Mail.app decides to switch to “Standard – Best for CRT” text rendering and make the text look optically correct, but behaviorally inconsistent with the rest of the operating system.
I may be wrong in this issue, but have anyone else had this problem?

June 22, 2009 at 1:28 pm
I’m also running 10.5.7 and can confirm this issue. “Sending…” used normal (non-thickened) text, but when it changed back to “Sent” the thickening reappeared.
June 22, 2009 at 1:38 pm
Exactly.
Another explanation for this phenomenon is if we assume that “Sending…” uses the regular weight of Lucida Grande, while “Sent” uses bold.
But even non-bold texts should not look that thin.
I wonder if we can submit a problem ticket on this issue?