Add a distinct sidebar panel background for vertical tab bars

Builds on top of kovidgoyal#9855 (vertical tab bars on left/right). When tab_bar_edge
is left or right and tab_bar_background is unset, derive a panel colour from
the theme (window background blended towards the foreground) so the sidebar
reads as a distinct panel rather than blending into the window content.
Overridable by setting tab_bar_background explicitly.

Also fill the area around the cell grid with the tab bar background instead
of the terminal background, so the panel stays one uniform colour to the
edges (no seam below/around the tabs).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: fawolfmann

kitty/config.py

@@ -141,6 +141,13 @@ def finalize_mouse_mappings(opts: Options, accumulate_bad_lines: list[BadLine] |
     opts.mousemap = mousemap
 
 
+def finalize_tab_bar(opts: Options) -> None:
+    from .fast_data_types import LEFT_EDGE, RIGHT_EDGE
+    if opts.tab_bar_background is None and opts.tab_bar_edge in (LEFT_EDGE, RIGHT_EDGE):
+        from .rgb import alpha_blend
+        opts.tab_bar_background = alpha_blend(opts.foreground, opts.background, 0.12)
+
+
 def parse_config(
     lines: Iterable[str], accumulate_bad_lines: list[BadLine] | None = None, effective_config_lines: Callable[[str, str], None] | None = None
 ) -> dict[str, Any]:
@@ -179,6 +186,7 @@ def add_effective_config_line(key: str, line: str) -> None:
     opts.alias_map.update(build_action_aliases(opts.action_alias))
     finalize_keys(opts, accumulate_bad_lines)
     finalize_mouse_mappings(opts, accumulate_bad_lines)
+    finalize_tab_bar(opts)
     # delete no longer needed definitions, replacing with empty placeholders
     opts.kitten_alias = {}
     opts.action_alias = {}

kitty/tab_bar.py

@@ -730,6 +730,8 @@ def update_blank_rects(self, central: Region, tab_bar: Region, vw: int, vh: int)
         opts = get_options()
         blank_rects: list[Border] = []
         bg = BorderColor.tab_bar_margin_color if opts.tab_bar_margin_color is not None else BorderColor.default_bg
+        if self.is_vertical and opts.tab_bar_margin_color is None and opts.tab_bar_background:
+            bg = BorderColor.tab_bar_bg
         if opts.tab_bar_margin_height:
             if self.is_vertical:
                 if opts.tab_bar_edge == LEFT_EDGE: