path: fix win32 relative for driveless root paths

lib/path.js

@@ -707,8 +707,17 @@ const win32 = {
     // We found a mismatch before the first common path separator was seen, so
     // return the original `to`.
     if (i !== length) {
-      if (lastCommonSep === -1)
-        return toOrig;
+      if (lastCommonSep === -1) {
+        // Two driveless rooted paths (e.g. `\\foo` and `\\bar`) share the
+        // current-drive root, so a relative path can still be produced even
+        // when their first segments differ. For any other roots (different
+        // drives, UNC vs. drive, etc.) no relative path exists.
+        if (fromStart === 1 && toStart === 1) {
+          lastCommonSep = 0;
+        } else {
+          return toOrig;
+        }
+      }
     } else {
       if (toLen > length) {
         if (StringPrototypeCharCodeAt(to, toStart + i) ===
@@ -717,9 +726,12 @@ const win32 = {
           // For example: from='C:\\foo\\bar'; to='C:\\foo\\bar\\baz'
           return StringPrototypeSlice(toOrig, toStart + i + 1);
         }
-        if (i === 2) {
+        if (i === 2 && fromStart === 0) {
           // We get here if `from` is the device root.
           // For example: from='C:\\'; to='C:\\foo'
+          // `fromStart === 0` ensures this only applies to device-rooted paths
+          // (e.g. `C:\\`) and not to driveless rooted paths (e.g. `\\foo`),
+          // where index 2 is regular path content rather than a device root.
           return StringPrototypeSlice(toOrig, toStart + i);
         }
       }
@@ -729,9 +741,11 @@ const win32 = {
           // We get here if `to` is the exact base path for `from`.
           // For example: from='C:\\foo\\bar'; to='C:\\foo'
           lastCommonSep = i;
-        } else if (i === 2) {
+        } else if (i === 2 && toStart === 0) {
           // We get here if `to` is the device root.
           // For example: from='C:\\foo\\bar'; to='C:\\'
+          // `toStart === 0` ensures this only applies to device-rooted paths
+          // (e.g. `C:\\`) and not to driveless rooted paths (e.g. `\\foo`).
           lastCommonSep = 3;
         }
       }
@@ -753,8 +767,17 @@ const win32 = {
 
     // Lastly, append the rest of the destination (`to`) path that comes after
     // the common path parts
-    if (out.length > 0)
+    if (out.length > 0) {
+      // The remaining `to` tail normally begins with the common path separator,
+      // which joins it to the `..` segments. For driveless rooted paths (e.g.
+      // `\\foo`) the leading separator was trimmed and `lastCommonSep` is the
+      // root, so insert the missing separator to avoid gluing `..` to the tail.
+      if (toStart < toEnd &&
+          StringPrototypeCharCodeAt(toOrig, toStart) !== CHAR_BACKWARD_SLASH) {
+        return `${out}\\${StringPrototypeSlice(toOrig, toStart, toEnd)}`;
+      }
       return `${out}${StringPrototypeSlice(toOrig, toStart, toEnd)}`;
+    }
 
     if (StringPrototypeCharCodeAt(toOrig, toStart) === CHAR_BACKWARD_SLASH)
       ++toStart;

test/parallel/test-path-relative.js

@@ -37,6 +37,23 @@ const relativeTests = [
      ['c:\\İ\\a\\i̇', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
      ['c:\\i̇\\a\\İ', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
      ['c:\\ß\\a\\ß', 'c:\\ß\\b\\ß\\test.txt', '..\\..\\b\\ß\\test.txt'],
+     // Driveless rooted paths (absolute relative to the current drive). These
+     // are structurally equivalent to POSIX paths and must not glue the `..`
+     // segments to the destination tail or trip the device-root shortcuts.
+     // Refs: https://github.com/nodejs/node/issues/63600
+     ['\\aaaa\\bbbb', '\\aaaa', '..'],
+     ['\\aaaa\\bbbb', '\\cccc', '..\\..\\cccc'],
+     ['\\aaaa\\bbbb', '\\aaaa\\bbbb', ''],
+     ['\\aaaa\\bbbb', '\\aaaa\\cccc', '..\\cccc'],
+     ['\\aaaa\\', '\\aaaa\\cccc', 'cccc'],
+     ['\\', '\\aaaa\\bbbb', 'aaaa\\bbbb'],
+     ['\\aa\\bb', '\\a', '..\\..\\a'],
+     ['\\aa', '\\a', '..\\a'],
+     ['\\abc\\d', '\\ab', '..\\..\\ab'],
+     ['\\ab', '\\abcd', '..\\abcd'],
+     ['\\foo\\bar', '\\f', '..\\..\\f'],
+     ['\\a', '\\a\\b\\c', 'b\\c'],
+     ['\\page1\\page2\\foo', '\\', '..\\..\\..'],
     ],
   ],
   [ path.posix.relative,