fix(api): make win_set_config with "win" for splits need "split/vertical"

Problem: currently, for splits, nvim_win_set_config accepts win without any of
split or vertical set, which has little effect and seems error-prone.

Solution: require at least one of split or vertical to also be set for splits.

Also, update nvim_win_set_config docs, as it's no longer limited to just
floating and external windows.

4 files changed, 27 insertions, 14 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 1b00777532..125976342b 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -3291,11 +3291,11 @@ nvim_win_get_config({window})                          *nvim_win_get_config()*
         Map defining the window configuration, see |nvim_open_win()|
 
 nvim_win_set_config({window}, {config})                *nvim_win_set_config()*
-    Configures window layout. Currently only for floating and external windows
-    (including changing a split window to those layouts).
+    Configures window layout. Cannot be used to move the last window in a
+    tabpage to a different one.
 
-    When reconfiguring a floating window, absent option keys will not be
-    changed. `row`/`col` and `relative` must be reconfigured together.
+    When reconfiguring a window, absent option keys will not be changed.
+    `row`/`col` and `relative` must be reconfigured together.
 
     Parameters: ~
       • {window}  Window handle, or 0 for current window
diff --git a/runtime/lua/vim/_meta/api.lua b/runtime/lua/vim/_meta/api.lua
index 94eab72291..4a179d49f3 100644
--- a/runtime/lua/vim/_meta/api.lua
+++ b/runtime/lua/vim/_meta/api.lua
@@ -2207,11 +2207,11 @@ function vim.api.nvim_win_remove_ns(window, ns_id) end
 --- @param buffer integer Buffer handle
 function vim.api.nvim_win_set_buf(window, buffer) end
 
---- Configures window layout. Currently only for floating and external windows
---- (including changing a split window to those layouts).
+--- Configures window layout. Cannot be used to move the last window in a
+--- tabpage to a different one.
 ---
---- When reconfiguring a floating window, absent option keys will not be
---- changed. `row`/`col` and `relative` must be reconfigured together.
+--- When reconfiguring a window, absent option keys will not be changed.
+--- `row`/`col` and `relative` must be reconfigured together.
 ---
 --- @param window integer Window handle, or 0 for current window
 --- @param config vim.api.keyset.win_config Map defining the window configuration, see `nvim_open_win()`
diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 557c2f37f9..21d6d59b1e 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -361,11 +361,11 @@ static int win_split_flags(WinSplit split, bool toplevel)
   return flags;
 }
 
-/// Configures window layout. Currently only for floating and external windows
-/// (including changing a split window to those layouts).
+/// Configures window layout. Cannot be used to move the last window in a
+/// tabpage to a different one.
 ///
-/// When reconfiguring a floating window, absent option keys will not be
-/// changed.  `row`/`col` and `relative` must be reconfigured together.
+/// When reconfiguring a window, absent option keys will not be changed.
+/// `row`/`col` and `relative` must be reconfigured together.
 ///
 /// @see |nvim_open_win()|
 ///
@@ -1099,11 +1099,15 @@ static bool parse_float_config(Dict(win_config) *config, WinConfig *fconfig, boo
         fconfig->window = config->win;
       }
     }
-  } else if (has_relative) {
-    if (HAS_KEY_X(config, win)) {
+  } else if (HAS_KEY_X(config, win)) {
+    if (has_relative) {
       api_set_error(err, kErrorTypeValidation,
                     "'win' key is only valid with relative='win' and relative=''");
       return false;
+    } else if (!is_split) {
+      api_set_error(err, kErrorTypeValidation,
+                    "non-float with 'win' requires at least 'split' or 'vertical'");
+      return false;
     }
   }
 
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index 30235d6b84..246ff50ddc 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -1727,6 +1727,15 @@ describe('API/win', function()
       config = api.nvim_win_get_config(win)
       eq('', config.relative)
       eq('below', config.split)
+
+      eq(
+        "non-float with 'win' requires at least 'split' or 'vertical'",
+        pcall_err(api.nvim_win_set_config, 0, { win = 0 })
+      )
+      eq(
+        "non-float with 'win' requires at least 'split' or 'vertical'",
+        pcall_err(api.nvim_win_set_config, 0, { win = 0, relative = '' })
+      )
     end)
 
     it('creates top-level splits', function()