Resizable child window show resize cursor when parent window is key?

>> I think it'd be a better UX if it did …

Statements like this make me cringe a little, because personal instincts (aka guesses) are notoriously unreliable. But anyway …

>> … since the child window is "attached"

… I can't parse this as a reason. What does the attachment (which presumably means here that the child moves when the parent moves) have to do with the resizing behavior? But anyway …

It sounds a bit like you're re-inventing drawers (NSDrawer). This UI component died on the vine because users hated it and found it hard to use. But anyway …

Without knowing what the child windows are for, it occurs to me that it might be easier to implement the child-resizing behavior in the parent window, as if that window had 2 frames, one that is grabbed to resize the main part, the other that is grabbed to resize the side parts. However, it's not clear whether your child windows look like windows, or can be re-positioned further away from the parent window as well as being resized, or can normally become key themselves. Those considerations probably affect your strategy for this.

FWIW.

>Statements like this make me cringe a little, because personal instincts (aka guesses) are notoriously unreliable

Well, the way I have this custom child window set up, it appears "attached" to the parent window, so IMO it appears to the user the the child window should get resize indicators when its parent is key and I hover my cursor over the edges of the child window in this case because they are "one" from the user's perspective.

I have my reasons for using an attached window instead a split view controller, though I don't feel the need to defend it so I won't. Just assume that I am qualified enough to make a decision about the UI in my app. And if you don't think I am, I don't care; I only care about my users. Intuition is not always a bad source BTW.

In an OSX App, I had to design aka child windows. At the end, I found it better and quite flexible to create floating windows (NSPanel) and manage the attachment by allowing to pin to the parent window.