zfsbootmenu-lib: show sort key where needed

When fzf supports a border label (>= 0.35.0), show the current sort key
that is being used to sort the list. This is only relevant on the main
BE selection screen and the Snapshot selection screen. If fzf does not
support a border label, fall back to legacy behavior of not showing the
value anywhere.

zfsbootmenu/lib/zfsbootmenu-lib.sh

@@ -124,7 +124,7 @@ global_header() {
 # returns: 0 on successful selection, 1 if Esc was pressed, 130 if BE list is missing
 
 draw_be() {
-  local env selected header expects kcl_text kcl_bind blank
+  local env selected header expects kcl_text kcl_bind blank sort_key preview_label
 
   env="${1}"
   if [ -z "${env}" ]; then
@@ -157,12 +157,17 @@ draw_be() {
 ^[CTRL+R] recovery shell
 ^[CTRL+H] help" )"
 
+  sort_key="$( get_sort_key )"
+  preview_label="Sorted by: ${sort_key^}"
+
   expects="--expect=alt-e,alt-k,alt-d,alt-s,alt-c,alt-r,alt-p,alt-w,alt-j,alt-o${kcl_bind:+,${kcl_bind}},right"
 
   # shellcheck disable=SC2086
   if ! selected="$( ${FUZZYSEL} -0 --prompt "BE > " \
       ${expects} ${expects//alt-/ctrl-} ${expects//alt-/ctrl-alt-} \
       ${HAS_BORDER:+--border-label="$( global_header )"} \
+      ${HAS_BORDER:+--preview-label-pos=2:bottom} \
+      ${HAS_BORDER:+--preview-label="$( colorize orange " ${preview_label} " )"} \
       --header="${header}" --preview-window="up:${PREVIEW_HEIGHT}${HAS_BORDER:+,border-sharp}" \
       --preview="/libexec/zfsbootmenu-preview {} ${BOOTFS}" < "${env}" )"; then
     return 1
@@ -230,7 +235,7 @@ draw_kernel() {
 # returns: 130 on error, 0 otherwise
 
 draw_snapshots() {
-  local benv selected header expects sort_key snapshots
+  local benv selected header expects sort_key snapshots note sorted_by context
 
   benv="${1}"
   if [ -z "${benv}" ]; then
@@ -239,8 +244,6 @@ draw_snapshots() {
   fi
   zdebug "using boot environment: ${benv}"
 
-  sort_key="$( get_sort_key )"
-
   header="$( column_wrap "\
 ^[RETURN] duplicate:[CTRL+C] clone only:[CTRL+X] clone and promote
 ^[CTRL+D] show diff:[CTRL+R] rollback:[CTRL+N] create new snapshot
@@ -251,7 +254,54 @@ draw_snapshots() {
 ^[CTRL+D] show diff
 ^[CTRL+H] help" )"
 
-  context="Note: for diff viewer, use tab to select/deselect up to two items"
+  sort_key="$( get_sort_key )"
+
+  sorted_by="Sorted by: ${sort_key^}"
+  note="Note: for diff viewer, use tab to select/deselect up to two items"
+
+  local LEGACY_CONTEXT
+  if [ -n "${HAS_BORDER}" ] ; then
+
+    # Determine how much space should be between the 'sorted by' text and a centered note
+    # Remove 4 extra characters so that we can put a 1 character pad between strings and 
+    # the horizontal box line
+
+    local spacer preview_offset
+
+    spacer=$(( ( ( COLUMNS - ${#note} ) / 2 ) - ${#sorted_by} - 4 ))
+
+    # preview_offset, if defined, controls the initial preview label text position
+    # refer to fzf documentation for the --preview-label-pos flag
+
+    # if spacer length is non-negative, everything fits
+    if [ "${spacer}" -gt 0 ]; then
+      preview_offset="2:"
+      printf -v spacer "%*s" "${spacer}" ""
+      # This is a unicode light solid line, U+2500
+      spacer="${spacer// /─}"
+      note="$( colorize orange "${note}" )"
+      sorted_by="$( colorize orange "${sorted_by}" )"
+      printf -v context " %s %s %s " "${sorted_by}" "${spacer}" "${note}"
+    # fall back to seeing if the note fits in the available columns
+    elif [ ${COLUMNS} -gt $(( ${#note} + 2 )) ]; then
+      printf -v context " %s " "$( colorize orange "${note}" )"
+    # very few screens will be narrower than this ...
+    elif [ ${COLUMNS} -gt $(( ${#sorted_by} +2 )) ]; then
+      preview_offset="2:"
+      printf -v context " %s " "$( colorize orange "${sorted_by}" )"
+    # this is a truly narrow screen, skip all preview label text
+    else
+      context=""
+    fi
+  else
+
+    # when defined this controls passing an additional parameter to zfsbootmenu-preview
+    # as well as extending the preview window height by 1
+    # when undefined, it triggers adding 0 to the window height, leaving it as-is
+
+    LEGACY_CONTEXT=1
+    context="${note}"
+  fi
 
   expects="--expect=alt-x,alt-c,alt-j,alt-o,alt-n,alt-r,left,right"
 
@@ -261,15 +311,6 @@ draw_snapshots() {
 
   zdebug "snapshots: ${snapshots[*]}"
 
-  # when defined this controls passing an additional parameter to zfsbootmenu-preview
-  # as well as extending the preview window height by 1
-  # when undefined, it triggers added 0 to the window height, leaving it as-is
-
-  local LEGACY_CONTEXT
-  if [ -z "${HAS_BORDER}" ]; then
-    LEGACY_CONTEXT=1
-  fi
-
   if ! selected="$(\
       HELP_SECTION=snapshot-management ${FUZZYSEL} \
         --prompt "Snapshot > " --header="${header}" --tac --multi 2 \
@@ -278,8 +319,8 @@ draw_snapshots() {
         --bind="alt-d:execute[ /libexec/zfsbootmenu-diff {+} ]${HAS_REFRESH:++refresh-preview}" \
         --bind="ctrl-d:execute[ /libexec/zfsbootmenu-diff {+} ]${HAS_REFRESH:++refresh-preview}" \
         --bind="ctrl-alt-d:execute[ /libexec/zfsbootmenu-diff {+} ]${HAS_REFRESH:++refresh-preview}" \
-        ${HAS_BORDER:+--preview-label-pos=bottom} \
-        ${HAS_BORDER:+--preview-label="$( colorize orange " ${context} " )"} \
+        ${HAS_BORDER:+--preview-label-pos=${preview_offset:+${preview_offset}}bottom} \
+        ${HAS_BORDER:+--preview-label="${context}"} \
         --preview="/libexec/zfsbootmenu-preview ${benv} ${BOOTFS} ${LEGACY_CONTEXT:+\"${context}\"}" \
         --preview-window="up:$(( PREVIEW_HEIGHT + ${LEGACY_CONTEXT:-0} ))${HAS_BORDER:+,border-sharp}" <<<"${snapshots}" )"
   then