Draw only the minimum borders needed for inactive windows

Fixes #699

Note that currently this is only implemented by the Tall layout.

docs/changelog.rst

@@ -6,6 +6,12 @@ Changelog
 0.11.3 [future]
 ------------------------------
 
+- Draw only the minimum borders needed for inactive windows. That is only the borders
+  that separate the inactive window from a neighbor. Note that setting
+  a non-zero window margin overrides this and causes all borders to be drawn.
+  The old behavior of drawing all borders can be restored via the
+  :opt:`draw_minimal_borders` setting in kitty.conf.
+
 - macOS: Add an option :opt:`macos_window_resizable` to control if kitty
   top-level windows are resizable using the mouse or not (:iss:`698`)
 

kitty/boss.py

@@ -25,6 +25,7 @@ from .fast_data_types import (
     set_clipboard_string, set_in_sequence_mode, toggle_fullscreen
 )
 from .keys import get_shortcut, shortcut_matches
+from .layout import set_draw_minimal_borders
 from .remote_control import handle_cmd
 from .rgb import Color, color_from_int
 from .session import create_session
@@ -76,6 +77,7 @@ class DumpCommands:  # {{{
 class Boss:
 
     def __init__(self, os_window_id, opts, args, cached_values, new_os_window_trigger):
+        set_draw_minimal_borders(opts)
         self.window_id_map = WeakValueDictionary()
         self.startup_colors = {k: opts[k] for k in opts if isinstance(opts[k], Color)}
         self.pending_sequences = None

kitty/config_data.py

@@ -449,6 +449,13 @@ o('window_border_width', 1.0, option_type=positive_float, long_text=_('''
 The width (in pts) of window borders. Will be rounded to the nearest number of pixels based on screen resolution.
 Note that borders are displayed only when more than one window is visible. They are meant to separate multiple windows.'''))
 
+o('draw_minimal_borders', True, long_text=_('''
+Draw only the minimum borders needed. This means that only the minimum
+needed borders for inactive windows are drawn. That is only the borders
+that separate the inactive window from a neighbor. Note that setting
+a non-zero window margin overrides this and causes all borders to be drawn.
+'''))
+
 o('window_margin_width', 0.0, option_type=positive_float, long_text=_('''
 The window margin (in pts) (blank area outside the border)'''))
 

kitty/layout.py

@@ -16,6 +16,7 @@ central = Region((0, 0, 199, 199, 200, 200))
 cell_width = cell_height = 20
 all_borders = True, True, True, True
 no_borders = False, False, False, False
+draw_minimal_borders = False
 
 
 def idx_for_id(win_id, windows):
@@ -28,6 +29,11 @@ def window_needs_borders(window, active_window):
     return window is active_window or window.needs_attention
 
 
+def set_draw_minimal_borders(opts):
+    global draw_minimal_borders
+    draw_minimal_borders = opts.draw_minimal_borders and opts.window_margin_width == 0
+
+
 def layout_dimension(start_at, length, cell_length, decoration_pairs, left_align=False, bias=None):
     number_of_windows = len(decoration_pairs)
     number_of_cells = length // cell_length
@@ -367,7 +373,10 @@ class Layout:  # {{{
         raise NotImplementedError()
 
     def resolve_borders(self, windows, active_window):
-        yield from self.do_resolve_borders(windows, active_window)
+        if draw_minimal_borders:
+            yield from self.do_resolve_borders(windows, active_window)
+        else:
+            yield from Layout.do_resolve_borders(self, windows, active_window)
 
     def do_resolve_borders(self, windows, active_window):
         for w in windows:
@@ -461,6 +470,26 @@ class Tall(Layout):  # {{{
         # left bottom blank rect
         self.bottom_blank_rect(windows[0])
 
+    def do_resolve_borders(self, windows, active_window):
+        only_bottom_border = False, False, False, True
+        last_i = len(windows) - 1
+        for i, w in enumerate(windows):
+            if window_needs_borders(w, active_window):
+                yield all_borders
+                continue
+            if i == 0:
+                if last_i == 1 and window_needs_borders(windows[1], active_window):
+                    yield no_borders
+                else:
+                    yield False, False, True, False
+                continue
+            if i == last_i:
+                yield no_borders
+                break
+            if active_window is windows[i+1] or windows[i+1].needs_attention:
+                yield no_borders
+            else:
+                yield only_bottom_border
 # }}}