Support negative inactive_text_alpha values for active-window-only fading

kitty/conf/utils.py

@@ -67,6 +67,10 @@ def unit_float(x: ConvertibleToNumbers) -> float:
     return max(0, min(float(x), 1))
 
 
+def signed_unit_float(x: ConvertibleToNumbers) -> float:
+    return max(-1, min(float(x), 1))
+
+
 def number_with_unit(x: str, default_unit: str, *extra_units: str) -> tuple[float, str]:
     if (mat := number_unit_pat.match(x)) is not None:
         try:

kitty/options/definition.py

@@ -1397,10 +1397,13 @@ opt('bell_border_color', '#ff5a00',
     )
 
 opt('inactive_text_alpha', '1.0',
-    option_type='unit_float', ctype='float',
+    option_type='signed_unit_float', ctype='float',
     long_text='''
-Fade the text in inactive windows by the specified amount (a number between zero
-and one, with zero being fully faded).
+Fade the text in inactive windows by the specified amount. This must be a
+number between negative one and one. The absolute value controls the actual
+opacity, with zero being fully faded and one being fully opaque. Negative
+values cause fading to be applied based only on whether the current window is
+active, ignoring the extra single-window unfocused case.
 '''
     )
 

kitty/options/parse.py

@@ -4,8 +4,8 @@
 import typing
 import collections.abc  # noqa: F401, RUF100
 from kitty.conf.utils import (
-    merge_dicts, positive_float, positive_int, python_string, to_bool, to_cmdline, to_color,
-    to_color_or_none, unit_float
+    merge_dicts, positive_float, positive_int, python_string, signed_unit_float, to_bool, to_cmdline,
+    to_color, to_color_or_none, unit_float
 )
 from kitty.options.utils import (
     action_alias, active_tab_title_template, allow_hyperlinks, bell_on_tab, box_drawing_scale,
@@ -1045,7 +1045,7 @@ class Parser:
         ans['inactive_tab_foreground'] = to_color(val)
 
     def inactive_text_alpha(self, val: str, ans: dict[str, typing.Any]) -> None:
-        ans['inactive_text_alpha'] = unit_float(val)
+        ans['inactive_text_alpha'] = signed_unit_float(val)
 
     def initial_window_height(self, val: str, ans: dict[str, typing.Any]) -> None:
         ans['initial_window_height'] = window_size(val)

kitty/shaders.c

@@ -1405,9 +1405,15 @@ draw_cells(const WindowRenderData *srd, OSWindow *os_window, bool is_active_wind
     bind_vertex_array(srd->vao_idx);
     // We draw with inactive text alpha if:
     // - We're not drawing the tab bar
-    // - There's only a single window and the os window is not focused
     // - There are multiple windows and the current window is not active
-    float current_inactive_text_alpha = is_tab_bar || (!is_single_window && is_active_window) || (is_single_window && screen->cursor_render_info.is_focused) ? 1.0f : (float)OPT(inactive_text_alpha);
+    // When inactive_text_alpha is negative, its absolute value is used as the
+    // opacity and fading is based only on whether the current window is active.
+    float configured_inactive_text_alpha = (float)OPT(inactive_text_alpha);
+    bool use_active_window_only = configured_inactive_text_alpha < 0.f;
+    if (configured_inactive_text_alpha < 0.f) configured_inactive_text_alpha = -configured_inactive_text_alpha;
+    float current_inactive_text_alpha = use_active_window_only ?
+        (is_tab_bar || is_active_window ? 1.0f : configured_inactive_text_alpha) :
+        (is_tab_bar || (!is_single_window && is_active_window) || (is_single_window && screen->cursor_render_info.is_focused) ? 1.0f : configured_inactive_text_alpha);
     float bg_alpha = effective_os_window_alpha(os_window);
 
     color_type default_bg = cell_update_uniform_block(

kitty_tests/options.py

@@ -311,6 +311,15 @@ def conf_parsing(self):
     self.ae(opts.kitty_mod, to_modifiers('alt'))
     self.ae(next(keys_for_func(opts, 'next_layout')).mods, opts.kitty_mod)
 
+    opts = p('inactive_text_alpha 0.25')
+    self.ae(opts.inactive_text_alpha, 0.25)
+    opts = p('inactive_text_alpha -0.25')
+    self.ae(opts.inactive_text_alpha, -0.25)
+    opts = p('inactive_text_alpha 2')
+    self.ae(opts.inactive_text_alpha, 1.0)
+    opts = p('inactive_text_alpha -2')
+    self.ae(opts.inactive_text_alpha, -1.0)
+
     # deprecation handling
     opts = p('clear_all_shortcuts y', 'send_text all f1 hello')
     self.ae(len(opts.keyboard_modes[''].keymap), 1)