feat(config-settings): add __config_settings_diff_redraw

functions/__config_settings_diff_redraw.fish

@@ -0,0 +1,47 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   __config_settings_diff_redraw <old_joined> <new_joined>
+#
+# DESCRIPTION
+#   Rewrites an on-screen panel frame in place, touching only the lines
+#   that changed. Assumes the cursor is already positioned at the top-left
+#   of the frame (the caller moves it there with a plain \e[<n>A -- no
+#   \e[J -- before calling this). Line count in old_joined and new_joined
+#   must be equal; a caller facing a height or width change should use the
+#   existing full erase+redraw path instead of calling this.
+#
+#   Unchanged lines advance the cursor with a bare newline, leaving
+#   whatever is already on screen untouched. Changed lines clear just that
+#   line (\e[2K), return to its start (\r), print the new content, and
+#   advance (\n). This is what removes the erase-then-redraw flicker: the
+#   screen is never blanked, only the handful of lines that actually
+#   differ are ever touched, and each of those is cleared and rewritten in
+#   the same breath rather than blanked-then-paused-then-filled.
+#
+# ARGUMENTS
+#   old_joined  Previous frame, lines joined with \n
+#   new_joined  New frame, lines joined with \n (same line count as old)
+#
+# EXIT STATUS
+#   0  Always
+#
+# RETURNS
+#   The ANSI sequence needed to turn the old frame into the new one,
+#   printed to stdout
+#
+# EXAMPLE
+#   __config_settings_diff_redraw (string join \n -- $prev_frame) \
+#       (string join \n -- $new_frame)
+function __config_settings_diff_redraw
+    set -l old (string split \n -- $argv[1])
+    set -l new (string split \n -- $argv[2])
+    for i in (seq (count $new))
+        if test "$old[$i]" = "$new[$i]"
+            printf '\n'
+        else
+            printf '\e[2K\r%s\n' $new[$i]
+        end
+    end
+end

tests/functional.fish

@@ -125,6 +125,22 @@ function test_draw_line_count_matches_panel_h
     end
 end
 
+function test_diff_redraw_unchanged_lines_are_bare_newlines
+    functions -q __config_settings_diff_redraw; or return 1
+    set -l old (string join \n -- AAA BBB CCC | string collect)
+    set -l new (string join \n -- AAA BBB CCC | string collect)
+    set -l out (__config_settings_diff_redraw "$old" "$new" | string collect -N)
+    test "$out" = \n\n\n
+end
+
+function test_diff_redraw_changed_line_is_cleared_and_rewritten
+    functions -q __config_settings_diff_redraw; or return 1
+    set -l old (string join \n -- AAA BBB CCC | string collect)
+    set -l new (string join \n -- AAA XYZ CCC | string collect)
+    set -l out (__config_settings_diff_redraw "$old" "$new" | string collect -N)
+    test "$out" = \n\e\[2K\rXYZ\n\n
+end
+
 function functional_test_main
     set -l names (functions -a | string match 'test_*' | sort)
     set -l failed 0