Preserve terminal-default foregrounds during brightness adjustment

src/prompt_toolkit/styles/style_transformation.py

@@ -173,7 +173,7 @@ def transform_attrs(self, attrs: Attrs) -> Attrs:
 
         # If a foreground color is given without a background color.
         no_background = not attrs.bgcolor or attrs.bgcolor == "default"
-        has_fgcolor = attrs.color and attrs.color != "ansidefault"
+        has_fgcolor = attrs.color and attrs.color not in ("default", "ansidefault")
 
         if has_fgcolor and no_background:
             # Calculate new RGB values.

tests/test_style_transformation.py

@@ -2,7 +2,7 @@
 
 import pytest
 
-from prompt_toolkit.styles import AdjustBrightnessStyleTransformation, Attrs
+from prompt_toolkit.styles import AdjustBrightnessStyleTransformation, Attrs, Style
 
 
 @pytest.fixture
@@ -50,3 +50,24 @@ def test_adjust_brightness_style_transformation(default_attrs):
 
     attrs = tr2.transform_attrs(default_attrs._replace(color="00ffaa"))
     assert attrs.color == "00ffaa"
+
+
+@pytest.mark.parametrize("color", [None, "", "default", "ansidefault"])
+@pytest.mark.parametrize("brightness", [(0.5, 1.0), (0.0, 0.5)])
+def test_adjust_brightness_preserves_default_foreground(
+    default_attrs, color, brightness
+):
+    attrs = default_attrs._replace(color=color, bold=True)
+    tr = AdjustBrightnessStyleTransformation(*brightness)
+
+    assert tr.transform_attrs(attrs) == attrs
+
+
+@pytest.mark.parametrize("style_string", ["fg:default", "fg:default bg:default"])
+def test_adjust_brightness_preserves_explicit_default_style(style_string):
+    style = Style.from_dict({"text": style_string})
+    attrs = style.get_attrs_for_style_str("class:text")
+    tr = AdjustBrightnessStyleTransformation(0.5, 1.0)
+
+    assert attrs.color == "default"
+    assert tr.transform_attrs(attrs) == attrs