diff --git a/tests/windows_encoded_roots.rs b/tests/windows_encoded_roots.rs
new file mode 100644
index 0000000..6d1f386
--- /dev/null
+++ b/tests/windows_encoded_roots.rs
@@ -0,0 +1,335 @@
+#![cfg(target_family = "windows")]
+
+use std::{
+  borrow::Cow,
+  ffi::{OsStr, OsString},
+  os::windows::ffi::{OsStrExt, OsStringExt},
+  path::{Component, Path, PathBuf, Prefix},
+};
+
+use sugar_path::SugarPath;
+
+const FIRST_INVALID_UNIT: u16 = 0xd800;
+const SECOND_INVALID_UNIT: u16 = 0xd801;
+
+#[derive(Clone, Copy, Debug)]
+enum RootSlot {
+  UncServer,
+  UncShare,
+  VerbatimUncServer,
+  VerbatimUncShare,
+  Device,
+  Verbatim,
+}
+
+#[derive(Clone, Copy)]
+struct RootCase {
+  name: &'static str,
+  slot: RootSlot,
+  root_before_unit: &'static str,
+  root_after_unit: &'static str,
+  folded_before_unit: &'static str,
+  folded_after_unit: &'static str,
+  lowercase_unicode_root: &'static str,
+  uppercase_unicode_root: &'static str,
+}
+
+const ROOT_CASES: &[RootCase] = &[
+  RootCase {
+    name: "UNC server",
+    slot: RootSlot::UncServer,
+    root_before_unit: r"\\SeRvEr-",
+    root_after_unit: r"\ShArE",
+    folded_before_unit: r"\\sErVeR-",
+    folded_after_unit: r"\sHaRe",
+    lowercase_unicode_root: r"\\ä\share",
+    uppercase_unicode_root: r"\\Ä\share",
+  },
+  RootCase {
+    name: "UNC share",
+    slot: RootSlot::UncShare,
+    root_before_unit: r"\\SeRvEr\ShArE-",
+    root_after_unit: "",
+    folded_before_unit: r"\\sErVeR\sHaRe-",
+    folded_after_unit: "",
+    lowercase_unicode_root: r"\\server\ä",
+    uppercase_unicode_root: r"\\server\Ä",
+  },
+  RootCase {
+    name: "verbatim UNC server",
+    slot: RootSlot::VerbatimUncServer,
+    root_before_unit: r"\\?\UNC\SeRvEr-",
+    root_after_unit: r"\ShArE",
+    folded_before_unit: r"\\?\UNC\sErVeR-",
+    folded_after_unit: r"\sHaRe",
+    lowercase_unicode_root: r"\\?\UNC\ä\share",
+    uppercase_unicode_root: r"\\?\UNC\Ä\share",
+  },
+  RootCase {
+    name: "verbatim UNC share",
+    slot: RootSlot::VerbatimUncShare,
+    root_before_unit: r"\\?\UNC\SeRvEr\ShArE-",
+    root_after_unit: "",
+    folded_before_unit: r"\\?\UNC\sErVeR\sHaRe-",
+    folded_after_unit: "",
+    lowercase_unicode_root: r"\\?\UNC\server\ä",
+    uppercase_unicode_root: r"\\?\UNC\server\Ä",
+  },
+  RootCase {
+    name: "device namespace",
+    slot: RootSlot::Device,
+    root_before_unit: r"\\.\PiPe-",
+    root_after_unit: "",
+    folded_before_unit: r"\\.\pIpE-",
+    folded_after_unit: "",
+    lowercase_unicode_root: r"\\.\ä",
+    uppercase_unicode_root: r"\\.\Ä",
+  },
+  RootCase {
+    name: "generic verbatim",
+    slot: RootSlot::Verbatim,
+    root_before_unit: r"\\?\GlObAl-",
+    root_after_unit: "",
+    folded_before_unit: r"\\?\gLoBaL-",
+    folded_after_unit: "",
+    lowercase_unicode_root: r"\\?\ä",
+    uppercase_unicode_root: r"\\?\Ä",
+  },
+];
+
+fn path_with_unit(before: &str, unit: u16, after: &str, tail: &str) -> PathBuf {
+  let mut wide: Vec<u16> = before.encode_utf16().collect();
+  wide.push(unit);
+  wide.extend(after.encode_utf16());
+  wide.extend(tail.encode_utf16());
+  PathBuf::from(OsString::from_wide(&wide))
+}
+
+fn path_with_tail(root: &str, tail: &str) -> PathBuf {
+  let mut wide: Vec<u16> = root.encode_utf16().collect();
+  wide.extend(tail.encode_utf16());
+  PathBuf::from(OsString::from_wide(&wide))
+}
+
+fn wide(path: &Path) -> Vec<u16> {
+  path.as_os_str().encode_wide().collect()
+}
+
+fn assert_wide_eq(actual: &Path, expected: &Path, context: &str) {
+  assert_eq!(wide(actual), wide(expected), "{context}");
+}
+
+fn root_identifier(path: &Path, slot: RootSlot) -> &OsStr {
+  let Some(Component::Prefix(prefix)) = path.components().next() else {
+    panic!("expected {slot:?} prefix, found {path:?}");
+  };
+
+  match (slot, prefix.kind()) {
+    (RootSlot::UncServer, Prefix::UNC(server, _)) => server,
+    (RootSlot::UncShare, Prefix::UNC(_, share)) => share,
+    (RootSlot::VerbatimUncServer, Prefix::VerbatimUNC(server, _)) => server,
+    (RootSlot::VerbatimUncShare, Prefix::VerbatimUNC(_, share)) => share,
+    (RootSlot::Device, Prefix::DeviceNS(device)) => device,
+    (RootSlot::Verbatim, Prefix::Verbatim(name)) => name,
+    (_, actual) => panic!("expected {slot:?} prefix, found {actual:?} in {path:?}"),
+  }
+}
+
+fn assert_slot_contains_unit(path: &Path, slot: RootSlot, unit: u16) {
+  assert!(
+    root_identifier(path, slot).encode_wide().any(|actual| actual == unit),
+    "expected {slot:?} identifier to contain {unit:#06x}: {path:?}",
+  );
+}
+
+fn assert_file_result(result: Cow<'_, Path>, context: &str) {
+  assert_wide_eq(&result, Path::new("file"), context);
+}
+
+fn assert_owned_target(result: Cow<'_, Path>, target: &Path, context: &str) {
+  let Cow::Owned(result) = result else {
+    panic!("{context}: expected an owned normalized target");
+  };
+  assert_wide_eq(&result, target, context);
+}
+
+fn assert_same_encoded_root(case: RootCase) {
+  let target =
+    path_with_unit(case.root_before_unit, FIRST_INVALID_UNIT, case.root_after_unit, r"\BASE\file");
+  let base =
+    path_with_unit(case.folded_before_unit, FIRST_INVALID_UNIT, case.folded_after_unit, r"\base");
+  assert_slot_contains_unit(&target, case.slot, FIRST_INVALID_UNIT);
+  assert_slot_contains_unit(&base, case.slot, FIRST_INVALID_UNIT);
+
+  assert_file_result(target.relative(&base), &format!("{} relative", case.name));
+  assert_file_result(
+    target.try_relative(&base).expect("absolute inputs do not need cwd"),
+    &format!("{} try_relative", case.name),
+  );
+  assert_file_result(
+    target.relative_with(&base, "not/absolute"),
+    &format!("{} relative_with", case.name),
+  );
+}
+
+fn assert_distinct_encoded_root(case: RootCase) {
+  let target_root =
+    path_with_unit(case.root_before_unit, FIRST_INVALID_UNIT, case.root_after_unit, "");
+  let base_root =
+    path_with_unit(case.root_before_unit, SECOND_INVALID_UNIT, case.root_after_unit, "");
+  assert_eq!(
+    target_root.to_string_lossy(),
+    base_root.to_string_lossy(),
+    "{}: the lossy oracle must be unable to distinguish the units",
+    case.name,
+  );
+
+  let target =
+    path_with_unit(case.root_before_unit, FIRST_INVALID_UNIT, case.root_after_unit, r"\BASE\file");
+  let base =
+    path_with_unit(case.root_before_unit, SECOND_INVALID_UNIT, case.root_after_unit, r"\base");
+  assert_slot_contains_unit(&target, case.slot, FIRST_INVALID_UNIT);
+  assert_slot_contains_unit(&base, case.slot, SECOND_INVALID_UNIT);
+
+  assert_owned_target(target.relative(&base), &target, &format!("{} relative", case.name));
+  assert_owned_target(
+    target.try_relative(&base).expect("absolute inputs do not need cwd"),
+    &target,
+    &format!("{} try_relative", case.name),
+  );
+  assert_owned_target(
+    target.relative_with(&base, "not/absolute"),
+    &target,
+    &format!("{} relative_with", case.name),
+  );
+}
+
+fn assert_non_ascii_root_is_not_folded(case: RootCase) {
+  let target = path_with_tail(case.lowercase_unicode_root, r"\BASE\file");
+  let base = path_with_tail(case.uppercase_unicode_root, r"\base");
+  let _ = root_identifier(&target, case.slot);
+  let _ = root_identifier(&base, case.slot);
+
+  assert_owned_target(target.relative(&base), &target, &format!("{} relative", case.name));
+  assert_owned_target(
+    target.try_relative(&base).expect("absolute inputs do not need cwd"),
+    &target,
+    &format!("{} try_relative", case.name),
+  );
+  assert_owned_target(
+    target.relative_with(&base, "not/absolute"),
+    &target,
+    &format!("{} relative_with", case.name),
+  );
+}
+
+#[test]
+fn encoded_root_identifiers_compare_only_ascii_case_and_exact_native_units() {
+  for &case in ROOT_CASES {
+    assert_same_encoded_root(case);
+    assert_distinct_encoded_root(case);
+    assert_non_ascii_root_is_not_folded(case);
+  }
+
+  let target = path_with_unit(r"\\.\NaMe-", FIRST_INVALID_UNIT, "", r"\BASE\file");
+  let base = path_with_unit(r"\\?\nAmE-", FIRST_INVALID_UNIT, "", r"\base");
+  assert_slot_contains_unit(&target, RootSlot::Device, FIRST_INVALID_UNIT);
+  assert_slot_contains_unit(&base, RootSlot::Verbatim, FIRST_INVALID_UNIT);
+  assert_owned_target(target.relative(&base), &target, "device versus verbatim relative");
+  assert_owned_target(
+    target.try_relative(&base).expect("absolute inputs do not need cwd"),
+    &target,
+    "device versus verbatim try_relative",
+  );
+  assert_owned_target(
+    target.relative_with(&base, "not/absolute"),
+    &target,
+    "device versus verbatim relative_with",
+  );
+}
+
+fn assert_absolutizes_exactly(result: Cow<'_, Path>, expected: &Path, context: &str) {
+  assert_wide_eq(&result, expected, context);
+}
+
+#[test]
+fn absolutization_preserves_encoded_root_identifiers() {
+  for &case in ROOT_CASES {
+    let clean = path_with_unit(
+      case.root_before_unit,
+      FIRST_INVALID_UNIT,
+      case.root_after_unit,
+      r"\base\file",
+    );
+    assert_slot_contains_unit(&clean, case.slot, FIRST_INVALID_UNIT);
+    assert_absolutizes_exactly(clean.absolutize(), &clean, &format!("{} clean strict", case.name));
+    assert_absolutizes_exactly(
+      clean.try_absolutize().expect("absolute input does not need cwd"),
+      &clean,
+      &format!("{} clean try", case.name),
+    );
+    assert_absolutizes_exactly(
+      clean.absolutize_with("not/absolute"),
+      &clean,
+      &format!("{} clean explicit", case.name),
+    );
+
+    let dirty = path_with_unit(
+      case.root_before_unit,
+      FIRST_INVALID_UNIT,
+      case.root_after_unit,
+      r"\base\.\file\",
+    );
+    let normalized = path_with_unit(
+      case.root_before_unit,
+      FIRST_INVALID_UNIT,
+      case.root_after_unit,
+      r"\base\file",
+    );
+    let Cow::Owned(strict) = dirty.absolutize() else {
+      panic!("{} dirty strict: expected owned result", case.name);
+    };
+    assert_wide_eq(&strict, &normalized, &format!("{} dirty strict", case.name));
+    let Cow::Owned(fallible) = dirty.try_absolutize().expect("absolute input does not need cwd")
+    else {
+      panic!("{} dirty try: expected owned result", case.name);
+    };
+    assert_wide_eq(&fallible, &normalized, &format!("{} dirty try", case.name));
+    let Cow::Owned(explicit) = dirty.absolutize_with("not/absolute") else {
+      panic!("{} dirty explicit: expected owned result", case.name);
+    };
+    assert_wide_eq(&explicit, &normalized, &format!("{} dirty explicit", case.name));
+
+    let cwd =
+      path_with_unit(case.root_before_unit, FIRST_INVALID_UNIT, case.root_after_unit, r"\base");
+    let input = Path::new(r"pkg\.\file");
+    let expected = path_with_unit(
+      case.root_before_unit,
+      FIRST_INVALID_UNIT,
+      case.root_after_unit,
+      r"\base\pkg\file",
+    );
+    let Cow::Owned(borrowed_cwd) = input.absolutize_with(cwd.as_path()) else {
+      panic!("{} borrowed cwd: expected owned result", case.name);
+    };
+    assert_wide_eq(&borrowed_cwd, &expected, &format!("{} borrowed cwd", case.name));
+    let Cow::Owned(owned_cwd) = input.absolutize_with(cwd) else {
+      panic!("{} owned cwd: expected owned result", case.name);
+    };
+    assert_wide_eq(&owned_cwd, &expected, &format!("{} owned cwd", case.name));
+
+    let cwd =
+      path_with_unit(case.root_before_unit, FIRST_INVALID_UNIT, case.root_after_unit, r"\base");
+    let root_relative = Path::new(r"\pkg\.\file\");
+    let expected =
+      path_with_unit(case.root_before_unit, FIRST_INVALID_UNIT, case.root_after_unit, r"\pkg\file");
+    let Cow::Owned(borrowed_cwd) = root_relative.absolutize_with(cwd.as_path()) else {
+      panic!("{} root-relative borrowed cwd: expected owned result", case.name);
+    };
+    assert_wide_eq(&borrowed_cwd, &expected, &format!("{} root-relative borrowed cwd", case.name));
+    let Cow::Owned(owned_cwd) = root_relative.absolutize_with(cwd) else {
+      panic!("{} root-relative owned cwd: expected owned result", case.name);
+    };
+    assert_wide_eq(&owned_cwd, &expected, &format!("{} root-relative owned cwd", case.name));
+  }
+}