[RDY] Add buffer information to tabline_update (#12481)

* Add buffer information to tabline_update

Most terminal implementations of the tabline display buffer and tab
information. Many neovim-qt users disable GuiTabline because it lacks
functionality provided in the terminal implementation.

The tabline_update event should include buffer information too, so client GUIs
can display rich useful tabs.

runtime/doc/ui.txt

@@ -631,11 +631,13 @@ Tabline Events							   *ui-tabline*
 
 Activated by the `ext_tabline` |ui-option|.
 
-["tabline_update", curtab, tabs]
+["tabline_update", curtab, tabs, curbuf, buffers]
 	Tabline was updated. UIs should present this data in a custom tabline
-	widget.
-	curtab:	  Current Tabpage
-	tabs:	  List of Dicts [{ "tab": Tabpage, "name": String }, ...]
+	widget. Note: options `curbuf` + `buffers` were added in API7.
+	curtab:   Current Tabpage
+	tabs:     List of Dicts [{ "tab": Tabpage, "name": String }, ...]
+	curbuf:   Current buffer handle.
+	buffers:  List of Dicts [{ "buffer": buffer handle, "name": String}, ...]
 
 ==============================================================================
 Cmdline Events							   *ui-cmdline*

src/nvim/api/ui_events.in.h

@@ -130,7 +130,8 @@ void popupmenu_hide(void)
 void popupmenu_select(Integer selected)
   FUNC_API_SINCE(3) FUNC_API_REMOTE_ONLY;
 
-void tabline_update(Tabpage current, Array tabs)
+void tabline_update(Tabpage current, Array tabs,
+                    Buffer current_buffer, Array buffers)
   FUNC_API_SINCE(3) FUNC_API_REMOTE_ONLY;
 
 void cmdline_show(Array content, Integer pos, String firstc, String prompt,

src/nvim/screen.c

@@ -7211,7 +7211,24 @@ void ui_ext_tabline_update(void)
 
     ADD(tabs, DICTIONARY_OBJ(tab_info));
   }
-  ui_call_tabline_update(curtab->handle, tabs);
+
+  Array buffers = ARRAY_DICT_INIT;
+  FOR_ALL_BUFFERS(buf) {
+    // Do not include unlisted buffers
+    if (!buf->b_p_bl) {
+      continue;
+    }
+
+    Dictionary buffer_info = ARRAY_DICT_INIT;
+    PUT(buffer_info, "buffer", BUFFER_OBJ(buf->handle));
+
+    get_trans_bufname(buf);
+    PUT(buffer_info, "name", STRING_OBJ(cstr_to_string((char *)NameBuff)));
+
+    ADD(buffers, DICTIONARY_OBJ(buffer_info));
+  }
+
+  ui_call_tabline_update(curtab->handle, tabs, curbuf->handle, buffers);
 }
 
 /*

test/functional/ui/tabline_spec.lua

@@ -4,14 +4,17 @@ local clear, command, eq = helpers.clear, helpers.command, helpers.eq
 
 describe('ui/ext_tabline', function()
   local screen
-  local event_tabs, event_curtab
+  local event_tabs, event_curtab, event_curbuf, event_buffers
 
   before_each(function()
     clear()
     screen = Screen.new(25, 5)
     screen:attach({rgb=true, ext_tabline=true})
-    function screen:_handle_tabline_update(curtab, tabs)
-      event_curtab, event_tabs = curtab, tabs
+    function screen:_handle_tabline_update(curtab, tabs, curbuf, buffers)
+      event_curtab = curtab
+      event_tabs = tabs
+      event_curbuf = curbuf
+      event_buffers = buffers
     end
   end)
 
@@ -45,4 +48,38 @@ describe('ui/ext_tabline', function()
       eq(expected_tabs, event_tabs)
     end}
   end)
+
+  it('buffer UI events', function()
+    local expected_buffers_initial= {
+      {buffer = { id = 1 }, name = '[No Name]'},
+    }
+
+    screen:expect{grid=[[
+      ^                         |
+      ~                        |
+      ~                        |
+      ~                        |
+                               |
+    ]], condition=function()
+      eq({ id = 1}, event_curbuf)
+      eq(expected_buffers_initial, event_buffers)
+    end}
+
+    command("badd another-buffer")
+    command("bnext")
+
+    local expected_buffers = {
+      {buffer = { id = 2 }, name = 'another-buffer'},
+    }
+    screen:expect{grid=[[
+      ^                         |
+      ~                        |
+      ~                        |
+      ~                        |
+                               |
+    ]], condition=function()
+      eq({ id = 2 }, event_curbuf)
+      eq(expected_buffers, event_buffers)
+    end}
+  end)
 end)