Add distinct controls for child frames' borders (Bug#45620)

The background of the 'child-frame-border' face instead of the
'internal-border' face now controls the color of child frames'
borders.

The 'child-frame-border-width' frame parameter is now used for the
width of child frames' borders instead of internal-border-width',
though we still fall back on using the latter if the former is not
set.

* doc/lispref/frames.texi (Frame Layout): Mention
'child-frame-border' and 'child-frame-border-width'.
(Layout Parameters): Mention 'child-frame-border-width'.
* etc/NEWS: Mention new face 'child-frame-border' and frame
parameter 'child-frame-border-width'.
* lisp/faces.el (child-frame-border): New face.
* src/dispextern.h (enum face_id): Add CHILD_FRAME_BORDER_FACE_ID.
* src/frame.c (Fframe_child_frame_border_width): New function.
(gui_report_frame_params): Add entry for Qchild_frame_border_width.
* src/frame.h (struct frame): New slot child_frame_border_width.
(FRAME_CHILD_FRAME_BORDER_WIDTH): New inlined function.
* src/nsfns.m (ns_set_child_frame_border_width): New function.
(Fx_create_frame): Handle Qchild_frame_border_width parameter.
(ns_frame_parm_handlers): Add ns_set_child_frame_border_width.
* src/nsterm.m (ns_clear_under_internal_border): Handle
CHILD_FRAME_BORDER_FACE_ID.
* src/w32fns.c (w32_clear_under_internal_border): Handle
CHILD_FRAME_BORDER_FACE_ID.
(w32_set_internal_border_width): New function.
(Fx_create_frame): Handle Qchild_frame_border_width parameter.
(w32_frame_parm_handlers): Add w32_set_child_frame_border_width.
* src/xfaces.c (lookup_basic_face, realize_basic_faces): Handle
CHILD_FRAME_BORDER_FACE_ID.
* src/xfns.c (x_set_child_frame_border_width): New function.
(Fx_create_frame): Handle Qchild_frame_border_width parameter.
(x_frame_parm_handlers): Add x_set_child_frame_border_width.
* src/xterm.c (x_clear_under_internal_border)
(x_after_update_window_line): Handle CHILD_FRAME_BORDER_FACE_ID.

1 files changed, 16 insertions, 3 deletions

diff --git a/doc/lispref/frames.texi b/doc/lispref/frames.texi
index 7f2a6f75422..ef1b661b2a0 100644
--- a/doc/lispref/frames.texi
+++ b/doc/lispref/frames.texi
@@ -694,9 +694,17 @@ parameter (@pxref{Management Parameters}).
 
 @item Internal Border
 The internal border is a border drawn by Emacs around the inner frame
-(see below).  Its width is specified by the @code{internal-border-width}
-frame parameter (@pxref{Layout Parameters}).  Its color is specified by
-the background of the @code{internal-border} face.
+(see below). The specification of its appearance depends on whether
+the given frame is a child frame (@pxref{Child Frames}) or not.
+
+For normal frames its width is specified by the @code{internal-border-width}
+frame parameter (@pxref{Layout Parameters}) and its color is specified by the
+background of the @code{internal-border} face.
+
+For child frames its width is specified by the @code{child-frame-border-width}
+frame parameter (but will use the the @code{internal-border-width} parameter as
+a fallback) and its color is specified by the background of the
+@code{child-frame-border} face.
 
 @item Inner Frame
 @cindex inner frame
@@ -1790,6 +1798,11 @@ The width in pixels of the frame's outer border (@pxref{Frame Geometry}).
 The width in pixels of the frame's internal border (@pxref{Frame
 Geometry}).
 
+@vindex child-frame-border-width@r{, a frame parameter}
+@item child-frame-border-width
+The width in pixels of the frame's internal border (@pxref{Frame
+Geometry}) if the given frame is a child frame (@pxref{Child Frames}).
+
 @vindex vertical-scroll-bars@r{, a frame parameter}
 @item vertical-scroll-bars
 Whether the frame has scroll bars (@pxref{Scroll Bars}) for vertical