fix(api): nvim_win_set_buf(0, 0) fails if 'winfixbuf' is set #31576

## Problem
With 'winfixbuf' enabled, `nvim_win_set_buf` and `nvim_set_current_buf` fail
even if targeting the already-current buffer.

    vim.wo.winfixbuf = true
    vim.api.nvim_win_set_buf(0, 0)
    vim.api.nvim_set_current_buf(0)

Solution:
Check for this condition.

3 files changed, 30 insertions, 12 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 1262af5e40..42fc21deac 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -888,7 +888,7 @@ void nvim_set_current_buf(Buffer buffer, Error *err)
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
-  if (!buf) {
+  if (!buf || curwin->w_buffer == buf) {
     return;
   }
 
diff --git a/src/nvim/api/window.c b/src/nvim/api/window.c
index ee7729ce81..f415415fa7 100644
--- a/src/nvim/api/window.c
+++ b/src/nvim/api/window.c
@@ -59,7 +59,7 @@ void nvim_win_set_buf(Window window, Buffer buffer, Error *err)
 {
   win_T *win = find_window_by_handle(window, err);
   buf_T *buf = find_buffer_by_handle(buffer, err);
-  if (!win || !buf) {
+  if (!win || !buf || win->w_buffer == buf) {
     return;
   }
 
diff --git a/test/functional/options/winfixbuf_spec.lua b/test/functional/options/winfixbuf_spec.lua
index 124f194b5a..5bed2fc72f 100644
--- a/test/functional/options/winfixbuf_spec.lua
+++ b/test/functional/options/winfixbuf_spec.lua
@@ -8,8 +8,8 @@ describe("Nvim API calls with 'winfixbuf'", function()
     clear()
   end)
 
-  it("Calling vim.api.nvim_win_set_buf with 'winfixbuf'", function()
-    local results = exec_lua([[
+  it('vim.api.nvim_win_set_buf on non-current buffer', function()
+    local ok = exec_lua([[
       local function _setup_two_buffers()
         local buffer = vim.api.nvim_create_buf(true, true)
 
@@ -23,16 +23,16 @@ describe("Nvim API calls with 'winfixbuf'", function()
 
       local other_buffer = _setup_two_buffers()
       local current_window = 0
-      local results, _ = pcall(vim.api.nvim_win_set_buf, current_window, other_buffer)
+      local ok, _ = pcall(vim.api.nvim_win_set_buf, current_window, other_buffer)
 
-      return results
+      return ok
     ]])
 
-    assert(results == false)
+    assert(not ok)
   end)
 
-  it("Calling vim.api.nvim_set_current_buf with 'winfixbuf'", function()
-    local results = exec_lua([[
+  it('vim.api.nvim_set_current_buf on non-current buffer', function()
+    local ok = exec_lua([[
       local function _setup_two_buffers()
         local buffer = vim.api.nvim_create_buf(true, true)
 
@@ -45,11 +45,29 @@ describe("Nvim API calls with 'winfixbuf'", function()
       end
 
       local other_buffer = _setup_two_buffers()
-      local results, _ = pcall(vim.api.nvim_set_current_buf, other_buffer)
+      local ok, _ = pcall(vim.api.nvim_set_current_buf, other_buffer)
 
-      return results
+      return ok
     ]])
 
-    assert(results == false)
+    assert(not ok)
+  end)
+
+  it('vim.api.nvim_win_set_buf on current buffer', function()
+    exec_lua([[
+      vim.wo.winfixbuf = true
+      local curbuf = vim.api.nvim_get_current_buf()
+      vim.api.nvim_win_set_buf(0, curbuf)
+      assert(vim.api.nvim_get_current_buf() == curbuf)
+    ]])
+  end)
+
+  it('vim.api.nvim_set_current_buf on current buffer', function()
+    exec_lua([[
+      vim.wo.winfixbuf = true
+      local curbuf = vim.api.nvim_get_current_buf()
+      vim.api.nvim_set_current_buf(curbuf)
+      assert(vim.api.nvim_get_current_buf() == curbuf)
+    ]])
   end)
 end)