feat(highlight): add FloatFooter highlight group

Problem: No clear separation of floating title and footer highlighting.

Solution: Add new `FloatFooter` highlight group.

runtime/doc/api.txt

@@ -479,6 +479,7 @@ to disable various visual features such as the 'number' column.
 Other highlight groups specific to floating windows:
 - |hl-FloatBorder| for window's border
 - |hl-FloatTitle| for window's title
+- |hl-FloatFooter| for window's footer
 
 Currently, floating windows don't support some widgets like scrollbar.
 
@@ -3146,7 +3147,7 @@ nvim_open_win({buffer}, {enter}, {*config})                  *nvim_open_win()*
                     Default is `"left"`.
                   • footer: Footer (optional) in window border, string or
                     list. List should consist of `[text, highlight]` tuples.
-                    If string, the default highlight group is `FloatTitle`.
+                    If string, the default highlight group is `FloatFooter`.
                   • footer_pos: Footer position. Must be set with `footer`
                     option. Value can be one of "left", "center", or "right".
                     Default is `"left"`.

runtime/doc/news.txt

@@ -156,7 +156,7 @@ The following new APIs and features were added.
   support fully MessagePack-RPC compliant clients.
 
 • Floating windows can now show footer with new `footer` and `footer_pos`
-  config fields.
+  config fields. Uses |hl-FloatFooter| by default.
 
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*

runtime/doc/syntax.txt

@@ -5273,6 +5273,8 @@ NormalFloat	Normal text in floating windows.
 FloatBorder	Border of floating windows.
 							*hl-FloatTitle*
 FloatTitle	Title of floating windows.
+							*hl-FloatFooter*
+FloatFooter	Footer of floating windows.
 							*hl-NormalNC*
 NormalNC	Normal text in non-current windows.
 							*hl-Pmenu*

runtime/doc/vim_diff.txt

@@ -258,6 +258,7 @@ Highlight groups:
   |hl-NormalFloat| highlights floating window
   |hl-FloatBorder| highlights border of a floating window
   |hl-FloatTitle| highlights title of a floating window
+  |hl-FloatFooter| highlights footer of a floating window
   |hl-NormalNC| highlights non-current windows
   |hl-MsgArea| highlights messages/cmdline area
   |hl-MsgSeparator| highlights separator for scrolled messages

runtime/lua/vim/_meta/api.lua

@@ -1559,7 +1559,7 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---                 Default is `"left"`.
 ---               • footer: Footer (optional) in window border, string or
 ---                 list. List should consist of `[text, highlight]` tuples.
----                 If string, the default highlight group is `FloatTitle`.
+---                 If string, the default highlight group is `FloatFooter`.
 ---               • footer_pos: Footer position. Must be set with `footer`
 ---                 option. Value can be one of "left", "center", or "right".
 ---                 Default is `"left"`.

src/nvim/api/win_config.c

@@ -153,7 +153,7 @@
 ///     Default is `"left"`.
 ///   - footer: Footer (optional) in window border, string or list.
 ///     List should consist of `[text, highlight]` tuples.
-///     If string, the default highlight group is `FloatTitle`.
+///     If string, the default highlight group is `FloatFooter`.
 ///   - footer_pos: Footer position. Must be set with `footer` option.
 ///     Value can be one of "left", "center", or "right".
 ///     Default is `"left"`.
@@ -428,16 +428,19 @@ static void parse_bordertext(Object bordertext, BorderTextType bordertext_type,
   bool *is_present;
   VirtText *chunks;
   int *width;
+  int default_hl_id;
   switch (bordertext_type) {
   case kBorderTextTitle:
     is_present = &fconfig->title;
     chunks = &fconfig->title_chunks;
     width = &fconfig->title_width;
+    default_hl_id = syn_check_group(S_LEN("FloatTitle"));
     break;
   case kBorderTextFooter:
     is_present = &fconfig->footer;
     chunks = &fconfig->footer_chunks;
     width = &fconfig->footer_width;
+    default_hl_id = syn_check_group(S_LEN("FloatFooter"));
     break;
   }
 
@@ -446,9 +449,8 @@ static void parse_bordertext(Object bordertext, BorderTextType bordertext_type,
       *is_present = false;
       return;
     }
-    int hl_id = syn_check_group(S_LEN("FloatTitle"));
     kv_push(*chunks, ((VirtTextChunk){ .text = xstrdup(bordertext.data.string.data),
-                                       .hl_id = hl_id }));
+                                       .hl_id = default_hl_id }));
     *width = (int)mb_string2cells(bordertext.data.string.data);
     *is_present = true;
     return;

src/nvim/highlight_group.c

@@ -173,6 +173,7 @@ static const char *highlight_init_both[] = {
   "default link NormalFloat Pmenu",
   "default link FloatBorder WinSeparator",
   "default link FloatTitle Title",
+  "default link FloatFooter Title",
   "default FloatShadow blend=80 guibg=Black",
   "default FloatShadowThrough blend=100 guibg=Black",
   "RedrawDebugNormal cterm=reverse gui=reverse",