diff --git a/testsuite/clean-fname-collapse_test.py b/testsuite/clean-fname-collapse_test.py
new file mode 100644
index 000000000..0d05b9272
--- /dev/null
+++ b/testsuite/clean-fname-collapse_test.py
@@ -0,0 +1,101 @@
+#!/usr/bin/env python3
+"""Regression test for clean_fname()'s ".." collapsing.
+
+clean_fname(name, CFN_COLLAPSE_DOT_DOT_DIRS) is rsync's canonical path
+normalizer.  Commit 9e089846 ("util: fixed issue in clean_fname()") fixed a
+read-before-buffer in the ".." back-up loop, but the new loop leaves 's'
+pointing at the *start* of the preceding component (just past its '/'),
+whereas the old loop left it *at* the '/'.  The surrounding test and
+assignment (`*s == '/'` / `t = s + 1`) were not adjusted for that one-byte
+shift, so the collapse silently stopped working for every path except a
+single leading no-slash component:
+
+    a/../b              -> b          (still worked)
+    /a/../b             -> /a/../b    (BROKEN: should be /b)
+    a/b/../c            -> a/b/../c   (BROKEN: should be a/c)
+    x/y/../../z         -> x/y/../../z(BROKEN: should be z)
+
+We exercise it through a merge-filter filename, one of the callers that pass
+CFN_COLLAPSE_DOT_DOT_DIRS.  The filter file lives at "a/c"; we reference it as
+"a/none/../c" where the "none" directory does NOT exist.  A correct collapse
+turns that into "a/c" (which opens), so the filter is read and applied; the
+buggy code leaves the literal "a/none/../c", which fails to open (ENOENT) and
+aborts the whole run.  This is also a faithful reproduction of the user-facing
+regression: merge/exclude files referenced via a path containing "..".
+"""
+
+import os
+
+from rsyncfns import (
+    FROMDIR, TODIR, rmtree, run_rsync, test_fail,
+)
+
+rmtree(FROMDIR)
+rmtree(TODIR)
+FROMDIR.mkdir(parents=True, exist_ok=True)
+TODIR.mkdir(parents=True, exist_ok=True)
+
+# Source: one file to keep, one the filter should drop.
+(FROMDIR / 'keep.txt').write_text('keep\n')
+(FROMDIR / 'drop.skip').write_text('drop\n')
+
+# Merge-filter file at a/c; note that a/none does NOT exist, so the path
+# "a/none/../c" only resolves if rsync collapses ".." to a plain "a/c".
+adir = TODIR.parent / 'a'
+rmtree(adir)
+adir.mkdir(parents=True, exist_ok=True)
+(adir / 'c').write_text('- *.skip\n')
+
+# Interior collapse only exercises the case where the ".." eats a component
+# that has a '/' before it.  Two more filter files pin the other two branches:
+# 'f1' for a leading component (the s == name case, where there is no earlier
+# '/' to find) and 'updir' for a leading ".." that must NOT be collapsed away.
+(TODIR.parent / 'f1').write_text('- *.skip\n')
+(TODIR.parent / 'updir').write_text('- *.skip\n')
+deepdir = TODIR.parent / 'sub1' / 'sub2'
+rmtree(deepdir)
+deepdir.mkdir(parents=True, exist_ok=True)
+
+
+def check(rundir, filter_path, what):
+    """Run with the merge filter named via `filter_path` and require that the
+    file was found *and* applied.  Each name only resolves when clean_fname()
+    collapses (or preserves) the ".." exactly right; get it wrong and the open
+    fails or lands on a different file, so nothing filters *.skip out."""
+    rmtree(TODIR)
+    TODIR.mkdir(parents=True, exist_ok=True)
+    saved = os.getcwd()
+    os.chdir(rundir)
+    try:
+        proc = run_rsync('-a', f'--filter=merge {filter_path}',
+                         f'{FROMDIR}/', f'{TODIR}/', check=False)
+    finally:
+        os.chdir(saved)
+
+    if proc.returncode != 0:
+        test_fail(f"rsync could not open the merge filter via '{filter_path}' "
+                  f"({what}); clean_fname() mishandled the '..'")
+    if not (TODIR / 'keep.txt').is_file():
+        test_fail(f"keep.txt was not transferred ({what})")
+    if (TODIR / 'drop.skip').exists():
+        test_fail(f"drop.skip was transferred ({what}): the merge filter was "
+                  "not applied, so clean_fname() resolved the filter path to "
+                  "the wrong file")
+
+
+# Interior component: "a/none/../c" -> "a/c" (a/none does not exist).
+check(TODIR.parent, 'a/none/../c', 'interior component')
+
+# Leading component: "zzz/../f1" -> "f1".  Here the back-up reaches the start
+# of the name with no '/' before it, so a bounds slip or an off-by-one in that
+# test stops the collapse (this is the shape the 9e089846 regression left
+# working, and the shape a partial revert breaks).
+check(TODIR.parent, 'zzz/../f1', 'leading component')
+
+# A leading ".." has nothing to collapse into and must be kept: run two levels
+# down and reach back up.  If it were collapsed away the name would resolve
+# inside sub1/sub2, where no such file exists.
+check(deepdir, '../../updir', 'leading ".." preserved')
+
+print("OK: clean_fname() collapsed interior and leading components and kept a "
+      "leading '..'; the merge filter applied in every case")
diff --git a/util1.c b/util1.c
index d9d9f4bcf..061f41baa 100644
--- a/util1.c
+++ b/util1.c
@@ -1004,13 +1004,16 @@ int clean_fname(char *name, int flags)
 					f += 2;
 					continue;
 				}
-				/* backing up for ".." — avoid reading before 'name' */
+				/* backing up for ".." — avoid reading before 'name'.
+				 * The loop stops with 's' pointing at the start of the
+				 * preceding component (just past its '/'), or at 'limit'. */
 				while (s > limit && s[-1] != '/')
 					s--;
 
-				/* If found prior '/', or we reached the start, adjust t. */
-				if (s != t - 1 && (s <= name || *s == '/')) {
-					t = (s == name) ? name : s + 1;
+				/* If we backed up over a real component (found a prior
+				 * '/' or reached the start of the name), drop it. */
+				if (s != t - 1 && (s == name || s[-1] == '/')) {
+					t = s;
 					f += 2;
 					continue;
 				}