Support default-font-* properties in Live-Preview

[tronical]

... by changing the resolution for the WindowItem to traverse the item tree from the current item, instead of going to the window.

This doesn't quite fix #4298 because rem resolution is still missing. That requires the built-in default font size function to be fixed as well, which is non-trivial.

cc #4298

[ogoffart]

why not self_rc: &ItemRc, like the other functions in this vtable have?

[tronical]

Ah yes, this is quite annoying. The issue is that &ItemRc translates to *const ItemRc in the C signature. This is the only function that's called from the C++ code straight into the vtable where a &ItemRc is needed. But because this takes a pointer, I can't write ...->layout_info(..., &ItemRc{{ ... }) as that's taking the address of a temporary. In all the other places in the FFI we also pass this "pair".

I could alternatively change this to &ItemRc but then on the C++ code call a helper function in our private header files that stores the ItemRc on the stack and passes the address. Would you prefer that?

[ogoffart]

I could alternatively change this to &ItemRc but then on the C++ code call a helper function in our private header files that stores the ItemRc on the stack and passes the address. Would you prefer that?

Yes, I think it is cleaner, don't you thik?

[tronical]

Yes, that's indeed cleaner.

[ogoffart]

No tests?

Could you make a test that involve a PopupWindow. I have a suspicion that it may not work because the PopupWindow inject a WindowItem that doesn't define these default-font-* properties.

eg:

export component TestCase inherits Window {
   default-font-size: 140px;
   PopupWindow {
      // the font size of this text must be large
      Text { text: "hello"; } 
   }
}
``

[tronical]

No tests?

Could you make a test that involve a PopupWindow. I have a suspicion that it may not work because the PopupWindow inject a WindowItem that doesn't define these default-font-* properties.

eg:

export component TestCase inherits Window {
   default-font-size: 140px;
   PopupWindow {
      // the font size of this text must be large
      Text { text: "hello"; } 
   }
}
``

That's an excellent point - this is indeed also not handled (besides the rem issue).