Treat stub blank lines in "if" as if top level

Resolves psf#2784

src/black/lines.py

@@ -530,10 +530,8 @@ def _maybe_empty_lines_for_class_or_def(
             return 0, 0
 
         if self.is_pyi:
-            if self.previous_line.depth > current_line.depth:
-                newlines = 0 if current_line.depth else 1
-            elif current_line.is_class or self.previous_line.is_class:
-                if current_line.depth:
+            if current_line.is_class:
+                if self.previous_line.depth < current_line.depth:
                     newlines = 0
                 elif current_line.is_stub_class and self.previous_line.is_stub_class:
                     # No blank line between classes with an empty body
@@ -551,6 +549,8 @@ def _maybe_empty_lines_for_class_or_def(
                     # Blank line between a block of functions (maybe with preceding
                     # decorators) and a block of non-functions
                     newlines = 1
+            elif self.previous_line.depth > current_line.depth:
+                newlines = 0 if current_line.depth else 1
             else:
                 newlines = 0
         else:

tests/data/stub.pyi

@@ -32,6 +32,15 @@ def g():
 
 def h(): ...
 
+if sys.version_info >= (3, 8):
+    class E:
+        def f(self): ...
+    class F:
+
+        def f(self): ...
+    class G: ...
+    class H: ...
+
 
 # output
 X: int
@@ -56,3 +65,13 @@ class A:
 
 def g(): ...
 def h(): ...
+
+if sys.version_info >= (3, 8):
+    class E:
+        def f(self): ...
+
+    class F:
+        def f(self): ...
+
+    class G: ...
+    class H: ...