Fix unstable subscript assignment string wrapping (#1678)

Fixes #1598

src/black/__init__.py

@@ -3507,9 +3507,12 @@ def __get_max_string_length(self, line: Line, string_idx: int) -> int:
                 # WMA4 a single space.
                 offset += 1
 
-                # WMA4 the lengths of any leaves that came before that space.
-                for leaf in LL[: p_idx + 1]:
+                # WMA4 the lengths of any leaves that came before that space,
+                # but after any closing bracket before that space.
+                for leaf in reversed(LL[: p_idx + 1]):
                     offset += len(str(leaf))
+                    if leaf.type in CLOSING_BRACKETS:
+                        break
 
         if is_valid_index(string_idx + 1):
             N = LL[string_idx + 1]

tests/data/long_strings__regression.py

@@ -349,6 +349,10 @@ def xxxxxxx_xxxxxx(xxxx):
             x.xxxxx.xxxxxx.xxxxx.xxxx,
         ], ("xxxxxxxxxxx xxxxxxx xxxx (xxxxxx xxxx) %x xxx xxxxx" % xxxxxxx_xxxx)
 
+value.__dict__[
+    key
+] = "test"  # set some Thrift field to non-None in the struct aa bb cc dd ee
+
 # output
 
 
@@ -784,3 +788,8 @@ def xxxxxxx_xxxxxx(xxxx):
         ], (
             "xxxxxxxxxxx xxxxxxx xxxx (xxxxxx xxxx) %x xxx xxxxx" % xxxxxxx_xxxx
         )
+
+
+value.__dict__[
+    key
+] = "test"  # set some Thrift field to non-None in the struct aa bb cc dd ee