Provide better completion for customizing frame parameters

* lisp/frame.el (frame--special-parameters): New const.
(frame--complete-parameter-value): New function.
(initial-frame-alist, minibuffer-frame-alist): Use them in
:type.  (Bug#39143)
* lisp/cus-start.el (default-frame-alist): Use them here as well.
* src/frame.c (frame_parms): Add comment to try to keep
frame--special-parameters updated.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/frame.c b/src/frame.c
index 1a5c2f5c3d4..b386839f34c 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -4392,6 +4392,10 @@ struct frame_parm_table {
   int sym;
 };
 
+/* If you're adding a new frame parameter here, consider if it makes sense
+   for the user to customize it via `initial-frame-alist' and the like.
+   If it does, add it to `frame--special-parameters' in frame.el, in order
+   to provide completion in the Customize UI for the new parameter.  */
 static const struct frame_parm_table frame_parms[] =
 {
   {"auto-raise",                SYMBOL_INDEX (Qauto_raise)},