Add directional drag-and-drop inserts for Vertical, Horizontal, Tall, Fat, Grid

Previously, body drops in all non-Splits layouts showed a full-window overlay
and performed a positional swap. This adds proper top/bottom or left/right
half-window overlays and true before/after insertion for the five layouts
Kovid identified.

Architecture:

- New `drag_overlay_mode` ClassVar on Layout ('full'|'axis_y'|'axis_x'|'free')
  controls both overlay display and valid direction axis. Layout subclasses set
  one line; tabs.py and boss.py dispatch on this attribute instead of hasattr.

- New `insert_window_group_next_to(target_group_id, after)` on WindowList
  performs a positional insert (not swap) by popping the active group and
  inserting it before or after the target.

- New base `insert_window_next_to` on Layout uses insert_window_group_next_to
  for axis_x/axis_y layouts and falls back to swap for 'full' (Stack).
  Splits overrides this with its existing tree-based implementation.

- `_insert_window_in_direction` in boss.py collapses from a 7-line hasattr
  branch to a single layout.insert_window_next_to() call.

Direction constraints:
  Vertical, Tall, Grid -> top/bottom (axis_y)
  Horizontal, Fat      -> left/right (axis_x)
  Splits               -> 4-way free (unchanged)
  Stack                -> full-window swap (unchanged)

kitty/boss.py

@@ -3340,13 +3340,7 @@ class Boss:
             layout = src_tab.current_layout
             horizontal = direction in ('left', 'right')
             after = direction in ('right', 'bottom')
-            if hasattr(layout, 'insert_window_next_to'):
-                layout.insert_window_next_to(src_tab.windows, window, dest_window, horizontal, after)
-            else:
-                wg_dest = src_tab.windows.group_for_window(dest_window)
-                if wg_dest:
-                    src_tab.windows.set_active_window_group_for(window)
-                    layout.move_window_to_group(src_tab.windows, wg_dest.id)
+            layout.insert_window_next_to(src_tab.windows, window, dest_window, horizontal, after)
             src_tab.relayout()
 
     def _move_tab_to(self, tab: Tab | None = None, target_os_window_id: int | None = None) -> Tab | None:

kitty/layout/base.py

@@ -4,7 +4,7 @@
 from collections.abc import Generator, Iterable, Iterator, Sequence
 from functools import partial
 from itertools import repeat
-from typing import Any, Callable, NamedTuple
+from typing import Any, Callable, ClassVar, Literal, NamedTuple
 
 from kitty.borders import BorderColor
 from kitty.fast_data_types import BOTTOM_EDGE, RIGHT_EDGE, Region, get_options, set_active_window, viewport_for_window
@@ -232,6 +232,12 @@ class Layout:
     must_draw_borders = False  # can be overridden to customize behavior from kittens
     layout_opts = LayoutOpts({})
     only_active_window_visible = False
+    # Controls drag-and-drop overlay display and valid direction axis for body drops.
+    # 'full'   – full-window overlay, positional swap (Stack and any unrecognised layout)
+    # 'axis_y' – top/bottom halves only (Vertical, Tall, Grid)
+    # 'axis_x' – left/right halves only (Horizontal, Fat)
+    # 'free'   – 4-way free direction (Splits; handled by its own insert_window_next_to override)
+    drag_overlay_mode: ClassVar[Literal['full', 'axis_y', 'axis_x', 'free']] = 'full'
 
     def __init__(self, os_window_id: int, tab_id: int, layout_opts: str = '') -> None:
         self.set_owner(os_window_id, tab_id)
@@ -303,6 +309,31 @@ class Layout:
     def move_window_to_group(self, all_windows: WindowList, group: int) -> bool:
         return all_windows.move_window_group(to_group=group)
 
+    def insert_window_next_to(
+        self,
+        all_windows: WindowList,
+        window: WindowType,
+        next_to: WindowType,
+        horizontal: bool,
+        after: bool,
+    ) -> None:
+        """Reposition window as a linear neighbour of next_to.
+
+        For axis_x/axis_y layouts this performs a positional insert that preserves
+        the order of all other groups. For 'full' layouts it falls back to a swap.
+        The Splits layout overrides this with tree-based logic.
+        """
+        src_wg = all_windows.group_for_window(window)
+        dest_wg = all_windows.group_for_window(next_to)
+        if src_wg is None or dest_wg is None or src_wg.id == dest_wg.id:
+            return
+        all_windows.set_active_window_group_for(window)
+        if self.drag_overlay_mode in ('axis_x', 'axis_y'):
+            all_windows.insert_window_group_next_to(dest_wg.id, after)
+        else:
+            # 'full' fallback: swap (preserves existing behaviour for Stack etc.)
+            self.move_window_to_group(all_windows, dest_wg.id)
+
     def add_window(
         self, all_windows: WindowList, window: WindowType, location: str | None = None,
         overlay_for: int | None = None, put_overlay_behind: bool = False, bias: float | None = None,

kitty/layout/grid.py

@@ -5,7 +5,7 @@ from collections.abc import Callable, Generator, Iterator, Sequence
 from functools import lru_cache
 from itertools import repeat
 from math import ceil, floor
-from typing import Any
+from typing import Any, ClassVar, Literal
 
 from kitty.borders import BorderColor
 from kitty.types import Edges, NeighborsMap, WindowMapper
@@ -34,6 +34,7 @@ class Grid(Layout):
 
     name: str = 'grid'
     no_minimal_window_borders = True
+    drag_overlay_mode: ClassVar[Literal['axis_y']] = 'axis_y'
 
     def remove_all_biases(self) -> bool:
         self.biased_rows: dict[int, float] = {}

kitty/layout/splits.py

@@ -2,7 +2,7 @@
 # License: GPLv3 Copyright: 2020, Kovid Goyal <kovid at kovidgoyal.net>
 
 from collections.abc import Collection, Generator, Iterator, Sequence
-from typing import Any, Optional, TypedDict, Union
+from typing import Any, ClassVar, Literal, Optional, TypedDict, Union
 
 from kitty.borders import BorderColor
 from kitty.fast_data_types import BOTTOM_EDGE, LEFT_EDGE, RIGHT_EDGE, TOP_EDGE
@@ -561,6 +561,7 @@ class Splits(Layout):
     needs_all_windows = True
     layout_opts = SplitsLayoutOpts({})
     no_minimal_window_borders = True
+    drag_overlay_mode: ClassVar[Literal['free']] = 'free'
 
     @property
     def default_axis_is_horizontal(self) -> bool | None:

kitty/layout/tall.py

@@ -4,7 +4,7 @@
 import sys
 from collections.abc import Generator, Iterator, Sequence
 from itertools import islice, repeat
-from typing import Any
+from typing import Any, ClassVar, Literal
 
 from kitty.borders import BorderColor
 from kitty.conf.utils import to_bool
@@ -136,6 +136,7 @@ class Tall(Layout):
     name = 'tall'
     main_is_horizontal = True
     no_minimal_window_borders = True
+    drag_overlay_mode: ClassVar[Literal['axis_y']] = 'axis_y'
     layout_opts = TallLayoutOpts({})
     main_axis_layout = Layout.xlayout
     perp_axis_layout = Layout.ylayout
@@ -381,5 +382,6 @@ class Fat(Tall):
 
     name = 'fat'
     main_is_horizontal = False
+    drag_overlay_mode: ClassVar[Literal['axis_x']] = 'axis_x'
     main_axis_layout = Layout.ylayout
     perp_axis_layout = Layout.xlayout

kitty/layout/vertical.py

@@ -2,7 +2,7 @@
 # License: GPLv3 Copyright: 2020, Kovid Goyal <kovid at kovidgoyal.net>
 
 from collections.abc import Generator, Iterable
-from typing import Any
+from typing import Any, ClassVar, Literal
 
 from kitty.borders import BorderColor
 from kitty.types import Edges, NeighborsMap, WindowMapper
@@ -64,6 +64,7 @@ class Vertical(Layout):
     name = 'vertical'
     main_is_horizontal = False
     no_minimal_window_borders = True
+    drag_overlay_mode: ClassVar[Literal['axis_y']] = 'axis_y'
     main_axis_layout = Layout.ylayout
     perp_axis_layout = Layout.xlayout
 
@@ -155,5 +156,6 @@ class Horizontal(Vertical):
 
     name = 'horizontal'
     main_is_horizontal = True
+    drag_overlay_mode: ClassVar[Literal['axis_x']] = 'axis_x'
     main_axis_layout = Layout.xlayout
     perp_axis_layout = Layout.ylayout

kitty/tabs.py

@@ -1950,18 +1950,25 @@ class TabManager:  # {{{
                     self._set_drag_target_window(dest_window.id, 5)
                     return
             active_tab = self.active_tab
-            if active_tab is not None and hasattr(active_tab.current_layout, 'insert_window_next_to'):
-                # Splits layout body hover: directional half-window overlay
+            if active_tab is not None:
+                mode = active_tab.current_layout.drag_overlay_mode
                 rel_x = x - central.left
                 g = dest_window.geometry
                 dx = rel_x - (g.left + g.right) / 2
                 dy = rel_y - (g.top + g.bottom) / 2
                 quad_map = {'left': 1, 'right': 2, 'top': 3, 'bottom': 4}
-                direction = ('right' if dx > 0 else 'left') if abs(dx) >= abs(dy) else ('bottom' if dy > 0 else 'top')
+                if mode == 'axis_y':
+                    direction = 'bottom' if dy > 0 else 'top'
+                elif mode == 'axis_x':
+                    direction = 'right' if dx > 0 else 'left'
+                elif mode == 'free':
+                    direction = ('right' if dx > 0 else 'left') if abs(dx) >= abs(dy) else ('bottom' if dy > 0 else 'top')
+                else:  # 'full' (Stack, etc.): full-window overlay, no directional highlight
+                    self._set_drag_target_window(dest_window.id, 6)
+                    return
                 self._set_drag_target_window(dest_window.id, quad_map[direction])
             else:
-                # All other body hover: full window overlay only (reorder/swap, no title bar flash)
-                self._set_drag_target_window(dest_window.id, 6)
+                self._set_drag_target_window(0)
         else:
             self._set_drag_target_window(0)
 
@@ -2024,13 +2031,16 @@ class TabManager:  # {{{
                 # Cross-tab title bar drop: move to the destination tab
                 boss._move_window_to(w, target_tab_id=active_tab.id)
         else:
-            # Quadrant-based directional insert
             g = dest_window.geometry
-            win_cx = (g.left + g.right) / 2
-            win_cy = (g.top + g.bottom) / 2
-            dx = rel_x - win_cx
-            dy = rel_y - win_cy
-            direction: str = ('right' if dx > 0 else 'left') if abs(dx) >= abs(dy) else ('bottom' if dy > 0 else 'top')
+            dx = rel_x - (g.left + g.right) / 2
+            dy = rel_y - (g.top + g.bottom) / 2
+            mode = active_tab.current_layout.drag_overlay_mode
+            if mode == 'axis_y':
+                direction: str = 'bottom' if dy > 0 else 'top'
+            elif mode == 'axis_x':
+                direction = 'right' if dx > 0 else 'left'
+            else:  # 'free' (Splits) or 'full' (swap fallback)
+                direction = ('right' if dx > 0 else 'left') if abs(dx) >= abs(dy) else ('bottom' if dy > 0 else 'top')
             boss._insert_window_in_direction(w, dest_window, direction)
 
     def update_progress(self) -> None:

kitty/window_list.py

@@ -519,6 +519,27 @@ class WindowList:
             return True
         return False
 
+    def insert_window_group_next_to(self, target_group_id: int, after: bool) -> bool:
+        """Move the active window group immediately before or after target_group_id.
+
+        Unlike move_window_group (which swaps), this is a positional insert that
+        preserves the relative order of all other groups.
+        """
+        src_idx = self.active_group_idx
+        if src_idx < 0 or not self.groups:
+            return False
+        target_idx = next((i for i, g in enumerate(self.groups) if g.id == target_group_id), -1)
+        if target_idx < 0 or src_idx == target_idx:
+            return False
+        group = self.groups.pop(src_idx)
+        # All indices above src shift down by one after the pop
+        if src_idx < target_idx:
+            target_idx -= 1
+        insert_pos = target_idx + (1 if after else 0)
+        self.groups.insert(insert_pos, group)
+        self.set_active_group_idx(insert_pos)
+        return True
+
     def compute_needs_borders_map(self, draw_active_borders: bool) -> dict[int, bool]:
         ag = self.active_group
         return {gr.id: ((gr is ag and draw_active_borders) or gr.needs_attention) for gr in self.groups}