From 899437217d1205b850891a5eaf2bb58e085946b2 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Sun, 8 Jan 2023 16:07:24 +0100
Subject: [PATCH 1/2] BUG: Fix bug in putmask for CoW

---
 pandas/core/internals/managers.py      |  6 ++----
 pandas/tests/copy_view/test_methods.py | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index c4e869a3f6a45..b67419e79224c 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -384,10 +384,8 @@ def setitem(self: T, indexer, value) -> T:
         return self.apply("setitem", indexer=indexer, value=value)
 
     def putmask(self, mask, new, align: bool = True):
-        if (
-            using_copy_on_write()
-            and self.refs is not None
-            and not all(ref is None for ref in self.refs)
+        if using_copy_on_write() and any(
+            not self._has_no_reference_block(i) for i in range(len(self.blocks))
         ):
             # some reference -> copy full dataframe
             # TODO(CoW) this could be optimized to only copy the blocks that would
diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index 7deb5e50464d5..dfc004a3cc353 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -700,3 +700,18 @@ def test_squeeze(using_copy_on_write):
         # Without CoW the original will be modified
         assert np.shares_memory(series.values, get_array(df, "a"))
         assert df.loc[0, "a"] == 0
+
+
+def test_putmask(using_copy_on_write):
+    df = DataFrame({"a": [1, 2], "b": 1, "c": 2})
+    view = df[:]
+    df_orig = df.copy()
+    df[df == df] = 5
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(view, "a"), get_array(df, "a"))
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        # Without CoW the original will be modified
+        assert np.shares_memory(get_array(view, "a"), get_array(df, "a"))
+        assert view.iloc[0, 0] == 5

From a40cec37c457812e4a5e3a5e44667faff1407d17 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Sun, 8 Jan 2023 16:14:32 +0100
Subject: [PATCH 2/2] Add whatsnew and gh ref

---
 doc/source/whatsnew/v1.5.3.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v1.5.3.rst b/doc/source/whatsnew/v1.5.3.rst
index 0cb8796e3fb5d..677fc904e6bf0 100644
--- a/doc/source/whatsnew/v1.5.3.rst
+++ b/doc/source/whatsnew/v1.5.3.rst
@@ -27,6 +27,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
+- Bug in the Copy-on-Write implementation losing track of views when indexing a :class:`DataFrame` with another :class:`DataFrame` (:issue:`50630`)
 - Bug in :meth:`.Styler.to_excel` leading to error when unrecognized ``border-style`` (e.g. ``"hair"``) provided to Excel writers (:issue:`48649`)
 - Bug when chaining several :meth:`.Styler.concat` calls, only the last styler was concatenated (:issue:`49207`)
 - Fixed bug when instantiating a :class:`DataFrame` subclass inheriting from ``typing.Generic`` that triggered a ``UserWarning`` on python 3.11 (:issue:`49649`)