bug#55588: pgtk: child-frame-border-width cannot be set to nil

[Po Lu]

Florian Rommel <mail@florommel.de> writes:

> In pgtk, child-frame-border-width is set to 0 by default, and it cannot
> be set to nil.
> According to the manual it should be possible for child-frame-border-
> width to be nil. In such cases, the value of internal-border-width is
> used as the border width (see 30.4.3.4 "Layout Parameters"; and
> function FRAME_INTERNAL_BORDER_WIDTH in frame.h).
>
> The problem causes missing child-frame borders for code that depends on
> this behavior (e.g., lsp-doc-ui in the lsp-ui package), and it will
> cause errors for code that tries to set child-frame-border-width to
> nil.
>
> The attached patch fixes this. It essentially replicates the respective
> behavior of xfns.c.

I remember there was a reason internal (and child frame) borders don't
work quite right in child frames on PGTK.  That might've been fixed in
December, but I cannot remember.

Please let me look into this first.