From 38c18ad3a8f231664abc0d12a9c57e73f93fafc6 Mon Sep 17 00:00:00 2001
From: Barrett Ruth <62671086+barrettruth@users.noreply.github.com>
Date: Sun, 17 May 2026 10:54:59 -0400
Subject: [PATCH] fix(vimscript): normalize multi-slash heads in fnamemodify()
 #39805

Problem: `fnamemodify(..., ':h')` mishandles POSIX leading slash runs longer than `//`.

Solution: Collapse those slash runs to `/` before computing the head.
---
 src/nvim/eval/fs.c                            | 15 ++++++++
 .../functional/vimscript/fnamemodify_spec.lua | 36 +++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/src/nvim/eval/fs.c b/src/nvim/eval/fs.c
index c3809b5faf..8bf7414d8e 100644
--- a/src/nvim/eval/fs.c
+++ b/src/nvim/eval/fs.c
@@ -203,6 +203,21 @@ repeat:
     }
   }
 
+#ifndef MSWIN
+  if (src[*usedlen] == ':' && src[*usedlen + 1] == 'h') {
+    char *fname = *fnamep;
+    // POSIX reserves exactly "//"; longer slash runs are ordinary absolute paths.
+    // "///foo/bar" => "/foo"
+    // "//foo/bar" => "//foo"
+    if (fname[0] == '/' && fname[1] == '/' && fname[2] == '/') {
+      while (fname[1] == '/') {
+        fname++;
+      }
+      *fnamep = fname;
+    }
+  }
+#endif
+
   char *tail = path_tail(*fnamep);
   *fnamelen = strlen(*fnamep);
 
diff --git a/test/functional/vimscript/fnamemodify_spec.lua b/test/functional/vimscript/fnamemodify_spec.lua
index 7b581c225f..f3d160f9c5 100644
--- a/test/functional/vimscript/fnamemodify_spec.lua
+++ b/test/functional/vimscript/fnamemodify_spec.lua
@@ -109,6 +109,42 @@ describe('fnamemodify()', function()
     eq('.', fnamemodify('hello.txt', ':h'))
 
     eq_slashconvert('path/to', fnamemodify('path/to/hello.txt', ':h'))
+
+    if is_os('win') then
+      -- Current Windows behavior for slash-style UNC paths, such as "//foo/C$/".
+      eq('/', fnamemodify('/', ':h'))
+      eq('/', fnamemodify('/foo', ':h'))
+      eq('//', fnamemodify('//', ':h'))
+      eq('//', fnamemodify('//foo', ':h'))
+      eq('//foo', fnamemodify('//foo/bar', ':h'))
+      eq('//foo', fnamemodify('//foo///bar', ':h'))
+      eq('//foo', fnamemodify('//foo/C$', ':h'))
+      eq('//foo/C$', fnamemodify('//foo/C$/', ':h'))
+      eq('//foo/C$', fnamemodify('//foo/C$/bar', ':h'))
+      eq('///', fnamemodify('///', ':h'))
+      eq('////', fnamemodify('////', ':h'))
+      eq('///', fnamemodify('///foo', ':h'))
+      eq('///foo', fnamemodify('///foo/bar', ':h'))
+      eq('///foo', fnamemodify('///foo////bar', ':h'))
+    else
+      -- POSIX roots.
+      eq('/', fnamemodify('/', ':h'))
+      eq('/', fnamemodify('/foo', ':h'))
+
+      -- POSIX permits special handling for exactly two leading slashes.
+      eq('//', fnamemodify('//', ':h'))
+      eq('//', fnamemodify('//foo', ':h'))
+      eq('//foo', fnamemodify('//foo/bar', ':h'))
+      eq('//foo', fnamemodify('//foo///bar', ':h'))
+
+      -- More than two leading slashes are ordinary absolute paths.
+      eq('/', fnamemodify('///', ':h'))
+      eq('/', fnamemodify('////', ':h'))
+      eq('/', fnamemodify('///foo', ':h'))
+      eq('/foo', fnamemodify('///foo/bar', ':h'))
+      eq('/foo', fnamemodify('///foo////bar', ':h'))
+      eq('/foo', fnamemodify('////foo/bar', ':h'))
+    end
   end)
 
   it('handles :t', function()