The DC superhero Power Girl wears a white costume with a red cape. Her costume has a cleavage-displaying cutout. Is there an in-universe, tactical explanation for this? Out of universe, it just seems like basic comic book sexism, but has any comic book writer given an in-universe explanation for the cutout?
Apparently there have been a few different justifications for the outfit over the years. A particularly notable explanation, from JSA: Classified #2 (2005), is that the costume was intended to have a symbol on the chest, like Superman's, but she never came up with anything and just left it empty.
People always ask me why I have this hole right here. They think I'm showing off ... or just being lewd. But the first time I made this costume, I wanted to have a symbol like you.
I just... I couldn't think of anything. I thought, eventually, I'd figure it out - and close the hole.
But I haven't.
This is not really a tactical reason for having a cleavage window, but it is at least an in-universe explanation. Apparently she was suffering from something of an identity crisis during this period, so she found her lack of symbolism particularly poignant.
Not being a reader of the comics, I'm only aware of this particular explanation for the costume because the scene of Power Girl having a tearful monologue that emotionally "justifies" her outfit was widely mocked on the internet. I'd be very surprised if no comic has ever canonicalised the simple joke that it's there to distract the kind of opponents who are easily distracted by breasts on display - I have seen panels where that property is remarked upon in-universe, though not given as the actual reasoning for the design.
The story that I heard was that her cleavage was due to the artist Wally Wood. It seems that Wally was bored, so he kept increasing her breast size each issue until somebody complained. I think he found another project before anybody complained, but they kept the look.
The "breast window" just made the problem more obvious.