fix for cache (#3538)

tests/test_cache.py

@@ -259,3 +259,29 @@ def test_discard():
     cache.discard("key4")  # key that does not exist
 
     assert len(cache) == 2  # size should not change
+
+
+def test_discard_regression():
+    """Regression test for https://github.com/Textualize/textual/issues/3537"""
+
+    cache = LRUCache(maxsize=3)
+    cache[1] = "foo"
+    cache[2] = "bar"
+    cache[3] = "baz"
+    cache[4] = "egg"
+
+    assert cache.keys() == {2, 3, 4}
+
+    cache.discard(2)
+    assert cache.keys() == {3, 4}
+
+    cache[5] = "bob"
+    assert cache.keys() == {3, 4, 5}
+
+    cache.discard(5)
+    assert cache.keys() == {3, 4}
+
+    cache.discard(4)
+    cache.discard(3)
+
+    assert cache.keys() == set()