Mark first block before matching content as changed in line ranges mapping

CHANGES.md

@@ -14,6 +14,8 @@
 <!-- Changes that affect Black's stable style -->
 
 - Don't double-decode input, causing non-UTF-8 files to be corrupted (#4964)
+- Fix `--line-ranges` not marking the first block as changed when it contains
+  modifications before the first matching content (#4997)
 
 ### Preview style
 

src/black/ranges.py

@@ -492,7 +492,7 @@ def _calculate_lines_mappings(
                         original_end=block.a,
                         modified_start=1,
                         modified_end=block.b,
-                        is_changed_block=False,
+                        is_changed_block=True,
                     )
                 )
         else:

tests/test_ranges.py

@@ -183,6 +183,23 @@ def test_diffs(lines: list[tuple[int, int]], adjusted: list[tuple[int, int]]) ->
     assert adjusted == adjusted_lines(lines, original_source, modified_source)
 
 
+def test_first_block_changed() -> None:
+    """Changes in the first block (before any matching content) should be included."""
+    original_source = """\
+x = 1
+y = 2
+z = 3
+"""
+    modified_source = """\
+x = 10
+y = 2
+z = 3
+"""
+    # Line 1 was changed, so range (1, 1) should map to (1, 1) in the output.
+    result = adjusted_lines([(1, 1)], original_source, modified_source)
+    assert result == [(1, 1)]
+
+
 @pytest.mark.parametrize(
     "lines,sanitized",
     [