Add vertical tab bars on the left and right

Teach tab_bar_edge about left and right sidebars and route tab layout,
hit-testing, and drag/drop through the vertical axis when needed.

kitty_tests/options.py

@@ -220,6 +220,7 @@ def launcher(self):
 def conf_parsing(self):
     from kitty.config import defaults, load_config
     from kitty.constants import is_macos
+    from kitty.fast_data_types import LEFT_EDGE, RIGHT_EDGE
     from kitty.fonts import FontModification, ModificationType, ModificationUnit, ModificationValue
     from kitty.options.utils import to_modifiers
     bad_lines = []
@@ -254,6 +255,10 @@ def conf_parsing(self):
     self.ae(opts.url_excluded_characters, "'''")
     opts = p("url_excluded_characters abc'")
     self.ae(opts.url_excluded_characters, "abc'")
+    opts = p('tab_bar_edge left')
+    self.ae(opts.tab_bar_edge, LEFT_EDGE)
+    opts = p('tab_bar_edge right')
+    self.ae(opts.tab_bar_edge, RIGHT_EDGE)
     opts = p('clear_all_shortcuts y', 'map f1 next_window')
     self.ae(len(opts.keyboard_modes[''].keymap), 1)
     opts = p('clear_all_mouse_actions y', 'mouse_map left click ungrabbed mouse_click_url_or_select')

kitty_tests/tab_bar.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python
+# License: GPL v3 Copyright: 2026, Kovid Goyal <kovid at kovidgoyal.net>
+
+from unittest.mock import patch
+
+from kitty.fast_data_types import LEFT_EDGE, Region
+from kitty.tab_bar import TabBar, TabBarData
+
+from . import BaseTest
+
+
+def region(left: int, top: int, right: int, bottom: int) -> Region:
+    return Region((left, top, right, bottom, right - left, bottom - top))
+
+
+class DummyBoss:
+    class mappings:
+        current_keyboard_mode_name = ''
+
+    def tab_for_id(self, tab_id: int) -> None:
+        return None
+
+
+class TestTabBar(BaseTest):
+
+    def test_vertical_tab_bar_hit_testing(self) -> None:
+        self.set_options({
+            'tab_bar_edge': LEFT_EDGE,
+            'tab_bar_style': 'separator',
+            'tab_title_template': '{title}',
+        })
+        central = region(120, 0, 400, 160)
+        tab_bar = region(0, 0, 120, 160)
+        geometries: list[tuple[int, int, int, int]] = []
+        boss = DummyBoss()
+
+        with (
+            patch('kitty.tab_bar.cell_size_for_window', return_value=(10, 20)),
+            patch('kitty.tab_bar.viewport_for_window', return_value=(central, tab_bar, 400, 160, 10, 20)),
+            patch('kitty.tab_bar.set_tab_bar_render_data', side_effect=lambda *args: geometries.append(args[2:6])),
+            patch('kitty.tab_bar.get_boss', return_value=boss),
+        ):
+            tb = TabBar(1)
+            tb.layout()
+            tb.update((
+                TabBarData(title='one', tab_id=1, is_active=True),
+                TabBarData(title='two', tab_id=2),
+                TabBarData(title='three', tab_id=3),
+            ))
+
+        self.assertTrue(tb.is_vertical)
+        self.ae(geometries[-1], (0, 0, 120, 160))
+        self.ae(tb.drag_axis_coordinate(5, 35), 35)
+        self.ae(tb.tab_id_at(5, 10), 1)
+        self.ae(tb.tab_id_at(110, 35), 2)
+        self.ae(tb.tab_id_at(60, 55), 3)
+        self.ae(tb.tab_id_at(60, 95), 0)
+        self.ae(tb.tab_id_at(180, 10), 0)