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/boss.py

@@ -2001,7 +2001,7 @@ class Boss:
                             window.on_drop(drop)
                             break
         elif tab_bar.left <= x < tab_bar.right and tab_bar.top <= y < tab_bar.bottom:
-            if (tab_id := tm.tab_bar.tab_id_at(x)) and (tab := self.tab_for_id(tab_id)) and (w := tab.active_window):
+            if (tab_id := tm.tab_bar.tab_id_at(x, y)) and (tab := self.tab_for_id(tab_id)) and (w := tab.active_window):
                 w.on_drop(drop)
 
     def on_drag_source_finished(

kitty/mouse.c

@@ -987,6 +987,8 @@ mouse_region(bool detect_borders, bool detect_title_bar) {
     const bool in_central = mouse_in_region(&central);
     if (!in_central) {
         if (
+                (tab_bar.left < central.left && w->mouse_x < central.left) ||
+                (tab_bar.right > central.right && w->mouse_x >= central.right) ||
                 (tab_bar.top < central.top && w->mouse_y < central.top) ||
                 (tab_bar.bottom > central.bottom && w->mouse_y >= central.bottom)
            ) ans.in_tab_bar = true;

kitty/options/definition.py

@@ -1616,18 +1616,22 @@ agr('tabbar', 'Tab bar')
 
 opt('tab_bar_edge', 'bottom',
     option_type='tab_bar_edge', ctype='int',
-    long_text='The edge to show the tab bar on, :code:`top` or :code:`bottom`.'
+    long_text='The edge to show the tab bar on, :code:`top`, :code:`bottom`, :code:`left` or :code:`right`.'
     )
 
 opt('tab_bar_margin_width', '0.0',
     option_type='positive_float',
-    long_text='The margin to the left and right of the tab bar (in pts).'
+    long_text='''
+The margin perpendicular to the tab bar edge (in pts). For tab bars on the
+top or bottom this is the margin to the left and right. For tab bars on the
+left or right this is the margin above and below.
+'''
     )
 
 opt('tab_bar_margin_height', '0.0 0.0',
     option_type='tab_bar_margin_height', ctype='!tab_bar_margin_height',
     long_text='''
-The margin above and below the tab bar (in pts). The first number is the margin
+The margin along the tab bar edge (in pts). The first number is the margin
 between the edge of the OS Window and the tab bar. The second number is the
 margin between the tab bar and the contents of the current tab.
 '''
@@ -1678,7 +1682,8 @@ are automatically restricted to work only on matching tabs.
 opt('tab_bar_align', 'left',
     choices=('left', 'center', 'right'),
     long_text='''
-The horizontal alignment of the tab bar, can be one of: :code:`left`,
+The horizontal alignment of the tab bar. For vertical tab bars this controls the
+alignment of each tab title within the sidebar. Can be one of: :code:`left`,
 :code:`center`, :code:`right`.
 '''
     )
@@ -1746,10 +1751,12 @@ this is rendered.
     )
 
 opt('tab_title_max_length', '0',
-    option_type='positive_int',
+    option_type='positive_int', ctype='int',
     long_text='''
 The maximum number of cells that can be used to render the text in a tab.
-A value of zero means that no limit is applied.
+A value of zero means that no limit is applied. For vertical tab bars, kitty
+uses a default sidebar width sized for about twenty title cells when this is
+left unset.
 '''
     )
 

kitty/options/to-c-generated.h

@@ -1084,6 +1084,19 @@ convert_from_opts_tab_bar_edge(PyObject *py_opts, Options *opts) {
     Py_DECREF(ret);
 }
 
+static void
+convert_from_python_tab_title_max_length(PyObject *val, Options *opts) {
+    opts->tab_title_max_length = PyLong_AsLong(val);
+}
+
+static void
+convert_from_opts_tab_title_max_length(PyObject *py_opts, Options *opts) {
+    PyObject *ret = PyObject_GetAttrString(py_opts, "tab_title_max_length");
+    if (ret == NULL) return;
+    convert_from_python_tab_title_max_length(ret, opts);
+    Py_DECREF(ret);
+}
+
 static void
 convert_from_python_tab_bar_margin_height(PyObject *val, Options *opts) {
     tab_bar_margin_height(val, opts);
@@ -1655,6 +1668,8 @@ convert_opts_from_python_opts(PyObject *py_opts, Options *opts) {
     if (PyErr_Occurred()) return false;
     convert_from_opts_tab_bar_edge(py_opts, opts);
     if (PyErr_Occurred()) return false;
+    convert_from_opts_tab_title_max_length(py_opts, opts);
+    if (PyErr_Occurred()) return false;
     convert_from_opts_tab_bar_margin_height(py_opts, opts);
     if (PyErr_Occurred()) return false;
     convert_from_opts_tab_bar_style(py_opts, opts);

kitty/options/utils.py

@@ -751,7 +751,12 @@ def tab_separator(x: str) -> str:
 
 
 def tab_bar_edge(x: str) -> int:
-    return {'top': defines.TOP_EDGE, 'bottom': defines.BOTTOM_EDGE}.get(x.lower(), defines.BOTTOM_EDGE)
+    return {
+        'left': defines.LEFT_EDGE,
+        'top': defines.TOP_EDGE,
+        'right': defines.RIGHT_EDGE,
+        'bottom': defines.BOTTOM_EDGE,
+    }.get(x.lower(), defines.BOTTOM_EDGE)
 
 
 def tab_font_style(x: str) -> tuple[bool, bool]:

kitty/state.c

@@ -691,29 +691,76 @@ pyset_borders_rects(PyObject *self UNUSED, PyObject *args) {
 }
 
 
+static unsigned
+vertical_tab_bar_cols(const OSWindow *os_window, long margin_outer, long margin_inner) {
+    unsigned cell_width = MAX(1u, os_window->fonts_data->fcm.cell_width);
+    long available_width = (long)os_window->viewport_width - margin_outer - margin_inner;
+    if (available_width <= 0) return 0;
+    unsigned available_cols = MAX(1u, (unsigned)available_width / cell_width);
+    unsigned title_cols = OPT(tab_title_max_length) > 0 ? (unsigned)OPT(tab_title_max_length) : 20u;
+    unsigned desired_cols = title_cols + 8u;
+    unsigned soft_max = available_cols / 3u;
+    if (soft_max < 6u) soft_max = available_cols;
+    return MAX(1u, MIN(available_cols, MIN(desired_cols, MAX(1u, soft_max))));
+}
+
 void
 os_window_regions(const OSWindow *os_window, Region *central, Region *tab_bar) {
     if (!OPT(tab_bar_hidden) && os_window->num_tabs && !os_window->has_too_few_tabs) {
         long margin_outer = pt_to_px_for_os_window(OPT(tab_bar_margin_height.outer), os_window);
         long margin_inner = pt_to_px_for_os_window(OPT(tab_bar_margin_height.inner), os_window);
         central->left = 0; central->right = os_window->viewport_width;
-        unsigned tab_bar_height = os_window->fonts_data->fcm.cell_height + margin_inner + margin_outer;
+        central->top = 0; central->bottom = os_window->viewport_height;
         switch(OPT(tab_bar_edge)) {
-            case TOP_EDGE:
+            case TOP_EDGE: {
+                unsigned tab_bar_height = os_window->fonts_data->fcm.cell_height + margin_inner + margin_outer;
                 central->top = tab_bar_height;
                 central->bottom = os_window->viewport_height;
                 central->top = MIN(central->top, central->bottom);
                 tab_bar->top = margin_outer;
+                tab_bar->left = central->left; tab_bar->right = central->right;
+                tab_bar->bottom = tab_bar->top + os_window->fonts_data->fcm.cell_height;
                 break;
-            default:
+            }
+            case LEFT_EDGE: {
+                unsigned left_cols = vertical_tab_bar_cols(os_window, margin_outer, margin_inner);
+                if (!left_cols) {
+                    zero_at_ptr(tab_bar);
+                    return;
+                }
+                unsigned left_width = left_cols * os_window->fonts_data->fcm.cell_width;
+                central->left = MIN((long)(left_width + margin_inner + margin_outer), (long)central->right);
+                tab_bar->left = margin_outer;
+                tab_bar->right = tab_bar->left + left_width;
+                tab_bar->top = central->top;
+                tab_bar->bottom = central->bottom;
+                break;
+            }
+            case RIGHT_EDGE: {
+                unsigned right_cols = vertical_tab_bar_cols(os_window, margin_outer, margin_inner);
+                if (!right_cols) {
+                    zero_at_ptr(tab_bar);
+                    return;
+                }
+                unsigned right_width = right_cols * os_window->fonts_data->fcm.cell_width;
+                central->right = MAX(0, (long)os_window->viewport_width - (long)(right_width + margin_inner + margin_outer));
+                tab_bar->left = central->right + margin_inner;
+                tab_bar->right = tab_bar->left + right_width;
+                tab_bar->top = central->top;
+                tab_bar->bottom = central->bottom;
+                break;
+            }
+            default: {
+                unsigned tab_bar_height = os_window->fonts_data->fcm.cell_height + margin_inner + margin_outer;
                 central->top = 0;
                 long bottom = os_window->viewport_height - tab_bar_height;
                 central->bottom = MAX(0, bottom);
                 tab_bar->top = central->bottom + margin_inner;
+                tab_bar->left = central->left; tab_bar->right = central->right;
+                tab_bar->bottom = tab_bar->top + os_window->fonts_data->fcm.cell_height;
                 break;
+            }
         }
-        tab_bar->left = central->left; tab_bar->right = central->right;
-        tab_bar->bottom = tab_bar->top + os_window->fonts_data->fcm.cell_height;
     } else {
         zero_at_ptr(tab_bar);
         central->left = 0; central->top = 0; central->right = os_window->viewport_width;

kitty/state.h

@@ -110,6 +110,7 @@ typedef struct Options {
     bool dynamic_background_opacity;
     float inactive_text_alpha;
     Edge tab_bar_edge;
+    int tab_title_max_length;
     DisableLigature disable_ligatures;
     bool force_ltr;
     bool resize_in_steps;

kitty/tab_bar.py

@@ -17,8 +17,11 @@ from .fast_data_types import (
     BOTTOM_EDGE,
     DECAWM,
     Color,
+    LEFT_EDGE,
     Region,
+    RIGHT_EDGE,
     Screen,
+    TOP_EDGE,
     background_opacity_of,
     cell_size_for_window,
     get_boss,
@@ -99,6 +102,22 @@ def as_rgb(x: int) -> int:
     return (x << 8) | 2
 
 
+VERTICAL_EDGES = frozenset({LEFT_EDGE, RIGHT_EDGE})
+
+
+def is_vertical_edge(edge: int) -> bool:
+    return edge in VERTICAL_EDGES
+
+
+def edge_name(edge: int) -> EdgeLiteral:
+    return {
+        LEFT_EDGE: 'left',
+        TOP_EDGE: 'top',
+        RIGHT_EDGE: 'right',
+        BOTTOM_EDGE: 'bottom',
+    }.get(edge, 'bottom')
+
+
 @lru_cache
 def report_template_failure(template: str, e: str) -> None:
     log_error(f'Invalid tab title template: "{template}" with error: {e}')
@@ -338,7 +357,7 @@ def draw_tab_with_slant(
     extra_data: ExtraData
 ) -> int:
     orig_fg = screen.cursor.fg
-    left_sep, right_sep = ('', '') if draw_data.tab_bar_edge == 'top' else ('', '')
+    left_sep, right_sep = ('', '') if draw_data.tab_bar_edge in ('top', 'left') else ('', '')
     tab_bg = screen.cursor.bg
     slant_fg = as_rgb(color_as_int(draw_data.default_bg))
 
@@ -563,10 +582,18 @@ class CellRange(NamedTuple):
 
 class TabExtent(NamedTuple):
     tab_id: int
-    cell_range: CellRange
+    x: CellRange
+    y: CellRange = CellRange(0, 0)
 
-    def shifted(self, shift: int) -> 'TabExtent':
-        return TabExtent(self.tab_id, CellRange(self.cell_range.start + shift, self.cell_range.end + shift))
+    def shifted(self, x: int = 0, y: int = 0) -> 'TabExtent':
+        return TabExtent(
+            self.tab_id,
+            CellRange(self.x.start + x, self.x.end + x),
+            CellRange(self.y.start + y, self.y.end + y),
+        )
+
+    def contains(self, x: int, y: int) -> bool:
+        return self.x.start <= x <= self.x.end and self.y.start <= y <= self.y.end
 
 
 class TabBar:
@@ -584,6 +611,8 @@ class TabBar:
     def apply_options(self) -> None:
         opts = get_options()
         self.dirty = True
+        self.tab_bar_edge = opts.tab_bar_edge
+        self.is_vertical = is_vertical_edge(opts.tab_bar_edge)
         self.margin_width = pt_to_px(opts.tab_bar_margin_width, self.os_window_id)
         self.cell_width, cell_height = cell_size_for_window(self.os_window_id)
         if not hasattr(self, 'screen'):
@@ -614,7 +643,7 @@ class TabBar:
             opts.active_tab_title_template,
             opts.tab_activity_symbol,
             opts.tab_powerline_style,
-            'bottom' if opts.tab_bar_edge == BOTTOM_EDGE else 'top',
+            edge_name(opts.tab_bar_edge),
             opts.tab_title_max_length, self.os_window_id,
         )
         ts = opts.tab_bar_style
@@ -628,6 +657,12 @@ class TabBar:
             self.draw_func = load_custom_draw_tab()
         else:
             self.draw_func = draw_tab_with_fade
+        if opts.tab_bar_align == 'center':
+            self.align_factor = 2
+        elif opts.tab_bar_align == 'right':
+            self.align_factor = 1
+        else:
+            self.align_factor = 0
         if opts.tab_bar_align == 'center':
             self.align: Callable[[], None] = partial(self.align_with_factor, 2)
         elif opts.tab_bar_align == 'right':
@@ -686,51 +721,96 @@ class TabBar:
         blank_rects: list[Border] = []
         bg = BorderColor.tab_bar_margin_color if opts.tab_bar_margin_color is not None else BorderColor.default_bg
         if opts.tab_bar_margin_height:
-            if opts.tab_bar_edge == BOTTOM_EDGE:
+            if self.is_vertical:
+                if opts.tab_bar_edge == LEFT_EDGE:
+                    if opts.tab_bar_margin_height.outer:
+                        blank_rects.append(Border(0, 0, tab_bar.left, vh, bg))
+                    if opts.tab_bar_margin_height.inner:
+                        blank_rects.append(Border(tab_bar.right, 0, central.left, vh, bg))
+                else:
+                    if opts.tab_bar_margin_height.outer:
+                        blank_rects.append(Border(tab_bar.right, 0, vw, vh, bg))
+                    if opts.tab_bar_margin_height.inner:
+                        blank_rects.append(Border(central.right, 0, tab_bar.left, vh, bg))
+            elif opts.tab_bar_edge == BOTTOM_EDGE:
                 if opts.tab_bar_margin_height.outer:
                     blank_rects.append(Border(0, tab_bar.bottom, vw, vh, bg))
                 if opts.tab_bar_margin_height.inner:
                     blank_rects.append(Border(0, central.bottom, vw, tab_bar.top, bg))
-            else: # top
+            else:  # top
                 if opts.tab_bar_margin_height.outer:
                     blank_rects.append(Border(0, 0, vw, tab_bar.top, bg))
                 if opts.tab_bar_margin_height.inner:
                     blank_rects.append(Border(0, tab_bar.bottom, vw, central.top, bg))
         g = self.window_geometry
-        left_bg = right_bg = bg
-        if opts.tab_bar_margin_color is None and (
-                opacity := background_opacity_of(self.os_window_id)) is not None and opacity >= 1:
-            left_bg = BorderColor.tab_bar_left_edge_color
-            right_bg = BorderColor.tab_bar_right_edge_color
-        if g.left > 0:
-            blank_rects.append(Border(0, g.top, g.left, g.bottom, left_bg))
-        if g.right < vw:
-            blank_rects.append(Border(g.right, g.top, vw, g.bottom, right_bg))
+        if self.is_vertical:
+            if g.left > tab_bar.left:
+                blank_rects.append(Border(tab_bar.left, g.top, g.left, g.bottom, bg))
+            if g.right < tab_bar.right:
+                blank_rects.append(Border(g.right, g.top, tab_bar.right, g.bottom, bg))
+            if g.top > tab_bar.top:
+                blank_rects.append(Border(g.left, tab_bar.top, g.right, g.top, bg))
+            if g.bottom < tab_bar.bottom:
+                blank_rects.append(Border(g.left, g.bottom, g.right, tab_bar.bottom, bg))
+        else:
+            left_bg = right_bg = bg
+            if opts.tab_bar_margin_color is None and (
+                    opacity := background_opacity_of(self.os_window_id)) is not None and opacity >= 1:
+                left_bg = BorderColor.tab_bar_left_edge_color
+                right_bg = BorderColor.tab_bar_right_edge_color
+            if g.left > tab_bar.left:
+                blank_rects.append(Border(tab_bar.left, g.top, g.left, g.bottom, left_bg))
+            if g.right < tab_bar.right:
+                blank_rects.append(Border(g.right, g.top, tab_bar.right, g.bottom, right_bg))
+            if g.top > tab_bar.top:
+                blank_rects.append(Border(g.left, tab_bar.top, g.right, g.top, bg))
+            if g.bottom < tab_bar.bottom:
+                blank_rects.append(Border(g.left, g.bottom, g.right, tab_bar.bottom, bg))
         self.blank_rects = tuple(blank_rects)
 
     def layout(self) -> None:
         central, tab_bar, vw, vh, cell_width, cell_height = viewport_for_window(self.os_window_id)
-        if tab_bar.width < 2:
+        if self.is_vertical:
+            if tab_bar.width < cell_width or tab_bar.height < cell_height:
+                return
+        elif tab_bar.width < 2:
             return
         self.cell_width = cell_width
+        self.cell_height = cell_height
         s = self.screen
-        available_width = tab_bar.width - 2 * self.margin_width
-        ncells = max(4, available_width // cell_width)
-        s.resize(1, ncells)
-        s.reset_mode(DECAWM)
-        cell_area_width = ncells * cell_width
-        available_width_for_left_margin = max(0, tab_bar.width - self.margin_width - cell_area_width)
-        extra_width = max(0, tab_bar.width - 2 * self.margin_width - cell_area_width)
-        left_margin = min(self.margin_width + extra_width // 2, available_width_for_left_margin)
+        if self.is_vertical:
+            available_height = tab_bar.height - 2 * self.margin_width
+            nlines = max(1, available_height // cell_height)
+            ncols = max(1, tab_bar.width // cell_width)
+            s.resize(nlines, ncols)
+            s.reset_mode(DECAWM)
+            cell_area_height = nlines * cell_height
+            available_height_for_top_margin = max(0, tab_bar.height - self.margin_width - cell_area_height)
+            extra_height = max(0, tab_bar.height - 2 * self.margin_width - cell_area_height)
+            top_margin = min(self.margin_width + extra_height // 2, available_height_for_top_margin)
+            self.window_geometry = g = WindowGeometry(
+                tab_bar.left, tab_bar.top + top_margin, tab_bar.right, tab_bar.top + top_margin + cell_area_height, s.columns, s.lines)
+        else:
+            available_width = tab_bar.width - 2 * self.margin_width
+            ncells = max(4, available_width // cell_width)
+            s.resize(1, ncells)
+            s.reset_mode(DECAWM)
+            cell_area_width = ncells * cell_width
+            available_width_for_left_margin = max(0, tab_bar.width - self.margin_width - cell_area_width)
+            extra_width = max(0, tab_bar.width - 2 * self.margin_width - cell_area_width)
+            left_margin = min(self.margin_width + extra_width // 2, available_width_for_left_margin)
+            self.window_geometry = g = WindowGeometry(
+                left_margin, tab_bar.top, left_margin + cell_area_width, tab_bar.bottom, s.columns, s.lines)
         self.laid_out_once = True
-        self.window_geometry = g = WindowGeometry(
-            left_margin, tab_bar.top, left_margin + cell_area_width, tab_bar.bottom, s.columns, s.lines)
         self.update_blank_rects(central, tab_bar, vw, vh)
         set_tab_bar_render_data(self.os_window_id, self.screen, *g[:4])
 
     def update(self, data: Sequence[TabBarData]) -> None:
         if not self.laid_out_once:
             return
+        if self.is_vertical:
+            self.update_vertical(data)
+            return
         s = self.screen
         last_tab = data[-1] if data else None
         ed = ExtraData()
@@ -739,14 +819,14 @@ class TabBar:
         def draw_tab(i: int, tab: TabBarData, cell_ranges: list[TabExtent], max_tab_length: int) -> None:
             ed.prev_tab = data[i - 1] if i > 0 else None
             ed.next_tab = data[i + 1] if i + 1 < len(data) else None
-            s.cursor.bg = as_rgb(self.draw_data.tab_bg(t))
-            s.cursor.fg = as_rgb(self.draw_data.tab_fg(t))
-            s.cursor.bold, s.cursor.italic = self.active_font_style if t.is_active else self.inactive_font_style
+            s.cursor.bg = as_rgb(self.draw_data.tab_bg(tab))
+            s.cursor.fg = as_rgb(self.draw_data.tab_fg(tab))
+            s.cursor.bold, s.cursor.italic = self.active_font_style if tab.is_active else self.inactive_font_style
             before = s.cursor.x
-            end = self.draw_func(self.draw_data, s, t, before, max_tab_length, i + 1, t is last_tab, ed)
+            end = self.draw_func(self.draw_data, s, tab, before, max_tab_length, i + 1, tab is last_tab, ed)
             s.cursor.bg = s.cursor.fg = 0
-            cell_ranges.append(TabExtent(tab_id=tab.tab_id, cell_range=CellRange(before, end)))
-            if not ed.for_layout and t is not last_tab and s.cursor.x > s.columns - max_tab_lengths[i+1]:
+            cell_ranges.append(TabExtent(tab_id=tab.tab_id, x=CellRange(before, end)))
+            if not ed.for_layout and tab is not last_tab and s.cursor.x > s.columns - max_tab_lengths[i+1]:
                 # Stop if there is no space for next tab
                 s.cursor.x = s.columns - 2
                 s.cursor.bg = as_rgb(color_as_int(self.draw_data.default_bg))
@@ -797,24 +877,72 @@ class TabBar:
         self.align()
         update_tab_bar_edge_colors(self.os_window_id)
 
+    def update_vertical(self, data: Sequence[TabBarData]) -> None:
+        s = self.screen
+        self.last_laid_out_tabs = data
+        self.tab_extents = ()
+        s.cursor.x = s.cursor.y = 0
+        s.erase_in_display(2, False)
+        if not data:
+            return
+        max_tab_length = max(1, s.columns - 1)
+        rows_to_draw = min(len(data), s.lines)
+        draw_ellipsis = len(data) > s.lines and s.lines > 1
+        if draw_ellipsis:
+            rows_to_draw -= 1
+        cr: list[TabExtent] = []
+        for i, t in enumerate(data[:rows_to_draw]):
+            s.cursor.x = 0
+            s.cursor.y = i
+            s.cursor.bg = as_rgb(self.draw_data.tab_bg(t))
+            s.cursor.fg = as_rgb(self.draw_data.tab_fg(t))
+            s.cursor.bold, s.cursor.italic = self.active_font_style if t.is_active else self.inactive_font_style
+            end = self.draw_func(self.draw_data, s, t, 0, max_tab_length, i + 1, True, ExtraData())
+            self.align_row(i, end)
+            cr.append(TabExtent(tab_id=t.tab_id, x=CellRange(0, s.columns - 1), y=CellRange(i, i)))
+        if draw_ellipsis:
+            s.cursor.x = 0
+            s.cursor.y = s.lines - 1
+            s.cursor.bg = as_rgb(color_as_int(self.draw_data.default_bg))
+            s.cursor.fg = as_rgb(0xff0000)
+            s.draw('…')
+        self.tab_extents = tuple(cr)
+
     def align_with_factor(self, factor: int = 1) -> None:
         if not self.tab_extents:
             return
-        end = self.tab_extents[-1].cell_range[1]
+        end = self.tab_extents[-1].x.end
         if end < self.screen.columns - 1:
             shift = (self.screen.columns - end) // factor
             self.screen.cursor.x = 0
             self.screen.insert_characters(shift)
-            self.tab_extents = tuple(te.shifted(shift) for te in self.tab_extents)
+            self.tab_extents = tuple(te.shifted(x=shift) for te in self.tab_extents)
+
+    def align_row(self, row: int, end: int) -> None:
+        if not self.align_factor:
+            return
+        if end < self.screen.columns - 1:
+            shift = (self.screen.columns - end) // self.align_factor
+            if shift > 0:
+                self.screen.cursor.y = row
+                self.screen.cursor.x = 0
+                self.screen.insert_characters(shift)
 
     def destroy(self) -> None:
         self.screen.reset_callbacks()
         del self.screen
 
-    def tab_id_at(self, x: int) -> int:
+    def tab_id_at(self, x: int, y: int = 0) -> int:
         if self.laid_out_once:
-            x = (x - self.window_geometry.left) // self.cell_width
+            g = self.window_geometry
+            if not (g.left <= x < g.right and g.top <= y < g.bottom):
+                return 0
+            x = (x - g.left) // self.cell_width
+            y = (y - g.top) // self.cell_height
             for te in self.tab_extents:
-                if te.cell_range.start <= x <= te.cell_range.end:
+                if te.contains(x, y):
                     return te.tab_id
         return 0
+
+    def drag_axis_coordinate(self, x: int, y: int) -> int:
+        return y if self.is_vertical else x

kitty/tabs.py

@@ -1160,7 +1160,7 @@ class Tab:  # {{{
 class TabBeingDropped(NamedTuple):
     data: TabBarData
     tab_ids: Sequence[int] = ()
-    last_drop_move_x: int = -1
+    last_drop_move_coordinate: int = -1
 
 
 class WindowBeingDropped(NamedTuple):
@@ -1692,27 +1692,29 @@ class TabManager:  # {{{
             tab_data = tab.data_for_tab_bar(tab is get_boss().active_tab)
             if tab_id not in all_tabs:
                 all_tabs.append(tab_id)
-            _, _, start_x, _ = get_tab_being_dragged()
-            self.tab_being_dropped = TabBeingDropped(data=tab_data, tab_ids=all_tabs, last_drop_move_x=int(start_x))
-            mouse_moved_left = False
+            _, _, start_x, start_y = get_tab_being_dragged()
+            start_coordinate = self.tab_bar.drag_axis_coordinate(int(start_x), int(start_y))
+            self.tab_being_dropped = TabBeingDropped(data=tab_data, tab_ids=all_tabs, last_drop_move_coordinate=start_coordinate)
             force_update = True
-        if x == self.tab_being_dropped.last_drop_move_x and not force_update:
+        coordinate = self.tab_bar.drag_axis_coordinate(x, y)
+        if coordinate == self.tab_being_dropped.last_drop_move_coordinate and not force_update:
             return
-        mouse_moved_left = x < self.tab_being_dropped.last_drop_move_x
+        mouse_moved_towards_start = coordinate < self.tab_being_dropped.last_drop_move_coordinate
         old_tab_ids = self.tab_being_dropped.tab_ids
         idx_under_mouse = -1
-        if (tab_id_under_mouse := self.tab_bar.tab_id_at(x)):
+        if (tab_id_under_mouse := self.tab_bar.tab_id_at(x, y)):
             with suppress(Exception):
                 idx_under_mouse = old_tab_ids.index(tab_id_under_mouse)
         if idx_under_mouse < 0:
-            idx_under_mouse = 0 if x < 20 else len(old_tab_ids) - 1
+            start = self.tab_bar.window_geometry.top if self.tab_bar.is_vertical else self.tab_bar.window_geometry.left
+            idx_under_mouse = 0 if coordinate < start else len(old_tab_ids) - 1
         old_idx_under_mouse = old_tab_ids.index(tab_id)
-        idx_moved_left = old_idx_under_mouse > idx_under_mouse
+        idx_moved_towards_start = old_idx_under_mouse > idx_under_mouse
         new_tab_ids = old_tab_ids
-        if mouse_moved_left == idx_moved_left:
+        if mouse_moved_towards_start == idx_moved_towards_start:
             new_tab_ids = list(old_tab_ids)
             new_tab_ids[idx_under_mouse], new_tab_ids[old_idx_under_mouse] = new_tab_ids[old_idx_under_mouse], new_tab_ids[idx_under_mouse]
-        self.tab_being_dropped = self.tab_being_dropped._replace(last_drop_move_x=x, tab_ids=new_tab_ids)
+        self.tab_being_dropped = self.tab_being_dropped._replace(last_drop_move_coordinate=coordinate, tab_ids=new_tab_ids)
         if force_update or self.tab_being_dropped.tab_ids != old_tab_ids:
             self.layout_tab_bar()
 
@@ -1784,9 +1786,9 @@ class TabManager:  # {{{
                     self.recent_tab_bar_mouse_events.clear()
             return
 
-        tab_id_at_x = self.tab_bar.tab_id_at(int(x))
-        self.recent_tab_bar_mouse_events.add(button, modifiers, action, x, y, tab_id_at_x)
-        if tab_id_at_x < 0:  # synthetic tab (e.g. "+" new-tab button)
+        tab_id_at_pointer = self.tab_bar.tab_id_at(int(x), int(y))
+        self.recent_tab_bar_mouse_events.add(button, modifiers, action, x, y, tab_id_at_pointer)
+        if tab_id_at_pointer < 0:  # synthetic tab (e.g. "+" new-tab button)
             if self.recent_tab_bar_mouse_events.click_count(GLFW_MOUSE_BUTTON_LEFT) == 1:
                 self.new_tab()
                 self.recent_tab_bar_mouse_events.clear()
@@ -1794,7 +1796,7 @@ class TabManager:  # {{{
         drag_started = get_tab_being_dragged()[1]
         if drag_started:
             return
-        tab = self.tab_for_id(tab_id_at_x)
+        tab = self.tab_for_id(tab_id_at_pointer)
         if tab is None:
             if self.recent_tab_bar_mouse_events.click_count(GLFW_MOUSE_BUTTON_LEFT) == 2:
                 self.new_tab()
@@ -1954,7 +1956,7 @@ class TabManager:  # {{{
         tab_bar = viewport_for_window(self.os_window_id)[1]
         if tab_bar.left <= x < tab_bar.right and tab_bar.top <= y < tab_bar.bottom:
             self._set_drag_target_window(0)
-            self._set_drag_target_tab(self.tab_bar.tab_id_at(x))
+            self._set_drag_target_tab(self.tab_bar.tab_id_at(x, y))
             return
         self._set_drag_target_tab(0)
         dest_window = self._find_window_at(x, y)
@@ -2009,7 +2011,7 @@ class TabManager:  # {{{
         # Case 1: Drop on tab bar → move to that tab
         in_tab_bar = tab_bar.left <= x < tab_bar.right and tab_bar.top <= y < tab_bar.bottom
         if in_tab_bar:
-            if (tab_id := self.tab_bar.tab_id_at(x)) and (dest_tab := self.tab_for_id(tab_id)):
+            if (tab_id := self.tab_bar.tab_id_at(x, y)) and (dest_tab := self.tab_for_id(tab_id)):
                 boss._move_window_to(w, target_tab_id=dest_tab.id)
             else:
                 boss._move_window_to(w, target_tab_id='new')

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)