macOS: Implement background blurring

Uses a private API that allows us to control the amount of blurring.
While using a private API is obviously not ideal, it is used by both
iTerm.app and Apple's own Terminal.app, so hopefully it should stick
around. Fixes #6135

kitty/options/definition.py

@@ -1371,6 +1371,17 @@ config.
 '''
     )
 
+opt('background_blur', '0', option_type='int', ctype='int',
+    long_text='''
+Set to a positive value to enable background blur (blurring of the visuals
+behind a transparent window) on platforms that support it. Only takes effect
+when :opt:`background_opacity` is less than one. On macOS, this will also
+control the :italic:`blur radius` (amount of blurring). Setting it to too high
+a value will cause severe performance issues and/or rendering artifacts.
+Usually, values up to 64 work well. Note that this might cause performance issues,
+depending on how the platform implements it, so use with care.
+''')
+
 opt('background_image', 'none',
     option_type='config_or_absolute_path', ctype='!background_image',
     long_text='Path to a background image. Must be in PNG format.'

kitty/options/parse.py

@@ -65,6 +65,9 @@ class Parser:
     def background(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
         ans['background'] = to_color(val)
 
+    def background_blur(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
+        ans['background_blur'] = int(val)
+
     def background_image(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
         ans['background_image'] = config_or_absolute_path(val)
 

kitty/options/to-c-generated.h

@@ -733,6 +733,19 @@ convert_from_opts_background_opacity(PyObject *py_opts, Options *opts) {
     Py_DECREF(ret);
 }
 
+static void
+convert_from_python_background_blur(PyObject *val, Options *opts) {
+    opts->background_blur = PyLong_AsLong(val);
+}
+
+static void
+convert_from_opts_background_blur(PyObject *py_opts, Options *opts) {
+    PyObject *ret = PyObject_GetAttrString(py_opts, "background_blur");
+    if (ret == NULL) return;
+    convert_from_python_background_blur(ret, opts);
+    Py_DECREF(ret);
+}
+
 static void
 convert_from_python_background_image(PyObject *val, Options *opts) {
     background_image(val, opts);
@@ -1159,6 +1172,8 @@ convert_opts_from_python_opts(PyObject *py_opts, Options *opts) {
     if (PyErr_Occurred()) return false;
     convert_from_opts_background_opacity(py_opts, opts);
     if (PyErr_Occurred()) return false;
+    convert_from_opts_background_blur(py_opts, opts);
+    if (PyErr_Occurred()) return false;
     convert_from_opts_background_image(py_opts, opts);
     if (PyErr_Occurred()) return false;
     convert_from_opts_background_image_layout(py_opts, opts);

kitty/options/types.py

@@ -61,6 +61,7 @@ option_names = (  # {{{
  'allow_hyperlinks',
  'allow_remote_control',
  'background',
+ 'background_blur',
  'background_image',
  'background_image_layout',
  'background_image_linear',
@@ -480,6 +481,7 @@ class Options:
     allow_hyperlinks: int = 1
     allow_remote_control: choices_for_allow_remote_control = 'no'
     background: Color = Color(0, 0, 0)
+    background_blur: int = 0
     background_image: typing.Optional[str] = None
     background_image_layout: choices_for_background_image_layout = 'tiled'
     background_image_linear: bool = False
@@ -511,7 +513,7 @@ class Options:
     cursor_underline_thickness: float = 2.0
     default_pointer_shape: choices_for_default_pointer_shape = 'beam'
     detect_urls: bool = True
-    dim_opacity: float = 0.75
+    dim_opacity: float = 0.4
     disable_ligatures: int = 0
     draw_minimal_borders: bool = True
     dynamic_background_opacity: bool = False