feat(float): implement footer

Problem: Now way to show text at the bottom part of floating window
  border (a.k.a. "footer").

Solution: Allows `footer` and `footer_pos` config fields similar to
  `title` and `title_pos`.

4 files changed, 29 insertions, 10 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 343c63f4b0..53d4282ec5 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -3138,11 +3138,18 @@ nvim_open_win({buffer}, {enter}, {*config})                  *nvim_open_win()*
                       specified by character: [ ["+", "MyCorner"], ["x",
                       "MyBorder"] ].
 
-                  • title: Title (optional) in window border, String or list.
-                    List is [text, highlight] tuples. if is string the default
-                    highlight group is `FloatTitle`.
-                  • title_pos: Title position must set with title option.
-                    value can be of `left` `center` `right` default is left.
+                  • title: Title (optional) in window border, string or list.
+                    List should consist of `[text, highlight]` tuples. If
+                    string, the default highlight group is `FloatTitle`.
+                  • title_pos: Title position. Must be set with `title`
+                    option. Value can be one of "left", "center", or "right".
+                    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`.
+                  • footer_pos: Footer position. Must be set with `footer`
+                    option. Value can be one of "left", "center", or "right".
+                    Default is `"left"`.
                   • noautocmd: If true then no buffer-related autocommand
                     events such as |BufEnter|, |BufLeave| or |BufWinEnter| may
                     fire from calling this function.
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 56bdd07171..c28ccb191e 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -155,6 +155,9 @@ The following new APIs and features were added.
 • New RPC client type `msgpack-rpc` is added for `nvim_set_client_info` to
   support fully MessagePack-RPC compliant clients.
 
+• Floating windows can now show footer with new `footer` and `footer_pos`
+  config fields.
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 
diff --git a/runtime/lua/vim/_meta/api.lua b/runtime/lua/vim/_meta/api.lua
index fdf5016b68..7704a60cf8 100644
--- a/runtime/lua/vim/_meta/api.lua
+++ b/runtime/lua/vim/_meta/api.lua
@@ -1551,11 +1551,18 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---                   specified by character: [ ["+", "MyCorner"], ["x",
 ---                   "MyBorder"] ].
 ---
----               • title: Title (optional) in window border, String or list.
----                 List is [text, highlight] tuples. if is string the default
----                 highlight group is `FloatTitle`.
----               • title_pos: Title position must set with title option.
----                 value can be of `left` `center` `right` default is left.
+---               • title: Title (optional) in window border, string or list.
+---                 List should consist of `[text, highlight]` tuples. If
+---                 string, the default highlight group is `FloatTitle`.
+---               • title_pos: Title position. Must be set with `title`
+---                 option. Value can be one of "left", "center", or "right".
+---                 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`.
+---               • footer_pos: Footer position. Must be set with `footer`
+---                 option. Value can be one of "left", "center", or "right".
+---                 Default is `"left"`.
 ---               • noautocmd: If true then no buffer-related autocommand
 ---                 events such as `BufEnter`, `BufLeave` or `BufWinEnter` may
 ---                 fire from calling this function.
diff --git a/runtime/lua/vim/_meta/api_keysets.lua b/runtime/lua/vim/_meta/api_keysets.lua
index 941780202e..08c29ebe7a 100644
--- a/runtime/lua/vim/_meta/api_keysets.lua
+++ b/runtime/lua/vim/_meta/api_keysets.lua
@@ -108,6 +108,8 @@ error('Cannot require a meta file')
 --- @field border? any
 --- @field title? any
 --- @field title_pos? string
+--- @field footer? any
+--- @field footer_pos? string
 --- @field style? string
 --- @field noautocmd? boolean