fix: Stop moving multiline strings to a new line unless inside brackets

CHANGES.md

@@ -16,6 +16,7 @@
 
 - `if` guards in `case` blocks are now wrapped in parentheses when the line is too long.
   (#4269)
+- Stop moving multiline strings to a new line unless inside brackets (#4289)
 
 ### Configuration
 

src/black/lines.py

@@ -858,11 +858,13 @@ def is_line_short_enough(  # noqa: C901
                     return False
 
         if leaf.bracket_depth <= max_level_to_update and leaf.type == token.COMMA:
-            # Ignore non-nested trailing comma
+            # Inside brackets, ignore trailing comma
             # directly after MLS/MLS-containing expression
             ignore_ctxs: List[Optional[LN]] = [None]
             ignore_ctxs += multiline_string_contexts
-            if not (leaf.prev_sibling in ignore_ctxs and i == len(line.leaves) - 1):
+            if (line.inside_brackets or leaf.bracket_depth > 0) and (
+                i != len(line.leaves) - 1 or leaf.prev_sibling not in ignore_ctxs
+            ):
                 commas[leaf.bracket_depth] += 1
         if max_level_to_update != math.inf:
             max_level_to_update = min(max_level_to_update, leaf.bracket_depth)

tests/data/cases/preview_multiline_strings.py

@@ -175,6 +175,13 @@ def dastardly_default_value(
     "c"
 )
 
+assert some_var == expected_result, """
+test
+"""
+assert some_var == expected_result, f"""
+expected: {expected_result}
+actual: {some_var}"""
+
 # output
 """cow
 say""",
@@ -385,3 +392,10 @@ def dastardly_default_value(
 )
 
 this_will_also_become_one_line = "abc"  # comment
+
+assert some_var == expected_result, """
+test
+"""
+assert some_var == expected_result, f"""
+expected: {expected_result}
+actual: {some_var}"""