diff --git a/kittens/icat/main.py b/kittens/icat/main.py
index 038f4f96b..e2c2464b5 100755
--- a/kittens/icat/main.py
+++ b/kittens/icat/main.py
@@ -22,7 +22,6 @@ from ..tui.images import (
 )
 from ..tui.operations import clear_images_on_screen, serialize_gr_command
 
-screen_size = None
 OPTIONS = '''\
 --align
 type=choices
@@ -97,6 +96,16 @@ Do not print out anything to stdout during operation.
 '''
 
 
+screen_size = None
+
+
+def get_screen_size():
+    global screen_size
+    if screen_size is None:
+        screen_size = screen_size_function()
+    return screen_size()
+
+
 def options_spec():
     if not hasattr(options_spec, 'ans'):
         options_spec.ans = OPTIONS.format(
@@ -122,7 +131,7 @@ def calculate_in_cell_x_offset(width, cell_width, align):
 
 
 def set_cursor(cmd, width, height, align):
-    ss = screen_size()
+    ss = get_screen_size()
     cw = int(ss.width / ss.cols)
     num_of_cells_needed = int(ceil(width / cw))
     if num_of_cells_needed > ss.cols:
@@ -143,7 +152,7 @@ def set_cursor(cmd, width, height, align):
 
 def set_cursor_for_place(place, cmd, width, height, align):
     x = place.left + 1
-    ss = screen_size()
+    ss = get_screen_size()
     cw = int(ss.width / ss.cols)
     num_of_cells_needed = int(ceil(width / cw))
     cmd['X'] = calculate_in_cell_x_offset(width, cw, align)
@@ -189,7 +198,7 @@ def show(outfile, width, height, fmt, transmit_mode='t', align='center', place=N
 
 def process(path, args, is_tempfile):
     m = identify(path)
-    ss = screen_size()
+    ss = get_screen_size()
     available_width = args.place.width * (ss.width / ss.cols) if args.place else ss.width
     available_height = args.place.height * (ss.height / ss.rows) if args.place else 10 * m.height
     needs_scaling = m.width > available_width or m.height > available_height
@@ -334,7 +343,7 @@ def main(args=sys.argv):
         sys.stdin.close()
         sys.stdin = open(os.ctermid(), 'r')
 
-    screen_size = screen_size_function()
+    screen_size = get_screen_size()
     signal.signal(signal.SIGWINCH, lambda signum, frame: setattr(screen_size, 'changed', True))
     if screen_size().width == 0:
         if args.detect_support:
diff --git a/kitty/fonts.c b/kitty/fonts.c
index 8059685bd..a2a0b3a9e 100644
--- a/kitty/fonts.c
+++ b/kitty/fonts.c
@@ -543,6 +543,14 @@ in_symbol_maps(FontGroup *fg, char_type ch) {
 }
 
 
+// Decides which 'font' to use for a given cell.
+//
+// Possible results:
+// - NO_FONT
+// - MISSING_FONT
+// - BLANK_FONT
+// - BOX_FONT
+// - an index in the fonts list
 static inline ssize_t
 font_for_cell(FontGroup *fg, CPUCell *cpu_cell, GPUCell *gpu_cell, bool *is_fallback_font, bool *is_emoji_presentation) {
     *is_fallback_font = false;
@@ -558,6 +566,10 @@ START_ALLOW_CASE_RANGE
         case 0x2574 ... 0x259f:
         case 0xe0b0 ... 0xe0b4:
         case 0xe0b6:
+        case 0xe0b8: // 
+        case 0xe0ba: //   
+        case 0xe0bc: // 
+        case 0xe0be: //   
             return BOX_FONT;
         default:
             ans = in_symbol_maps(fg, cpu_cell->ch);
@@ -586,14 +598,15 @@ set_sprite(GPUCell *cell, sprite_index x, sprite_index y, sprite_index z) {
     cell->sprite_x = x; cell->sprite_y = y; cell->sprite_z = z;
 }
 
+// Gives a unique (arbitrary) id to a box glyph
 static inline glyph_index
 box_glyph_id(char_type ch) {
 START_ALLOW_CASE_RANGE
     switch(ch) {
         case 0x2500 ... 0x259f:
-            return ch - 0x2500;
+            return ch - 0x2500; // IDs from 0x00 to 0x9f
         case 0xe0b0 ... 0xe0d4:
-            return 0xa0 + ch - 0xe0b0;
+            return 0xa0 + ch - 0xe0b0; // IDs from 0xa0 to 0xc4
         default:
             return 0xff;
     }
diff --git a/kitty/fonts/box_drawing.py b/kitty/fonts/box_drawing.py
index 88f050a96..e87109612 100644
--- a/kitty/fonts/box_drawing.py
+++ b/kitty/fonts/box_drawing.py
@@ -2,6 +2,11 @@
 # vim:fileencoding=utf-8
 # License: GPL v3 Copyright: 2017, Kovid Goyal <kovid at kovidgoyal.net>
 
+#
+# NOTE: to add a new glyph, add an entry to the `box_chars` dict, then update
+# the functions `font_for_cell` and `box_glyph_id` in `kitty/fonts.c`.
+#
+
 import math
 from functools import partial as p
 from itertools import repeat
@@ -172,6 +177,22 @@ def triangle(buf, width, height, left=True):
     fill_region(buf, width, height, xlimits)
 
 
+def corner_triangle(buf, width, height, corner):
+    if corner == 'top-right' or corner == 'bottom-left':
+        diagonal_y = line_equation(0, 0, width - 1, height - 1)
+        if corner == 'top-right':
+            xlimits = [(0, diagonal_y(x)) for x in range(width)]
+        elif corner == 'bottom-left':
+            xlimits = [(diagonal_y(x), height - 1) for x in range(width)]
+    else:
+        diagonal_y = line_equation(width - 1, 0, 0, height - 1)
+        if corner == 'top-left':
+            xlimits = [(0, diagonal_y(x)) for x in range(width)]
+        elif corner == 'bottom-right':
+            xlimits = [(diagonal_y(x), height - 1) for x in range(width)]
+    fill_region(buf, width, height, xlimits)
+
+
 def antialiased_1px_line(buf, width, height, p1, p2):
     # Draw an antialiased line using the Wu algorithm
     x1, y1 = p1
@@ -552,6 +573,10 @@ box_chars = {
     '': [p(D, left=False)],
     '': [p(half_cross_line, which='tl'), p(half_cross_line, which='bl')],
     '': [p(half_cross_line, which='tr'), p(half_cross_line, which='br')],
+    '': [p(corner_triangle, corner='bottom-left')],
+    '': [p(corner_triangle, corner='bottom-right')],
+    '': [p(corner_triangle, corner='top-left')],
+    '': [p(corner_triangle, corner='top-right')],
     '═': [dhline],
     '║': [dvline],
     '╞': [vline, p(half_dhline, which='right')],
@@ -647,60 +672,60 @@ def render_missing_glyph(buf, width, height):
 
 
 def test_char(ch, sz=48):
-    # kitty +runpy "from kitty.fonts.box_drawing import test_char; import sys; test_char('XXX')"
+    # kitty +runpy "from kitty.fonts.box_drawing import test_char; test_char('XXX')"
     from .render import display_bitmap, setup_for_testing
     from kitty.fast_data_types import concat_cells, set_send_sprite_to_gpu
-    width, height = setup_for_testing('monospace', sz)[1:]
-    buf = bytearray(width * height)
-    try:
-        render_box_char(ch, buf, width, height)
+    with setup_for_testing('monospace', sz) as (_, width, height):
+        buf = bytearray(width * height)
+        try:
+            render_box_char(ch, buf, width, height)
 
-        def join_cells(*cells):
-            cells = tuple(bytes(x) for x in cells)
-            return concat_cells(width, height, False, cells)
+            def join_cells(*cells):
+                cells = tuple(bytes(x) for x in cells)
+                return concat_cells(width, height, False, cells)
 
-        rgb_data = join_cells(buf)
-        display_bitmap(rgb_data, width, height)
-        print()
-    finally:
-        set_send_sprite_to_gpu(None)
+            rgb_data = join_cells(buf)
+            display_bitmap(rgb_data, width, height)
+            print()
+        finally:
+            set_send_sprite_to_gpu(None)
 
 
 def test_drawing(sz=48, family='monospace'):
     from .render import display_bitmap, setup_for_testing
     from kitty.fast_data_types import concat_cells, set_send_sprite_to_gpu
 
-    width, height = setup_for_testing(family, sz)[1:]
-    space = bytearray(width * height)
+    with setup_for_testing(family, sz) as (_, width, height):
+        space = bytearray(width * height)
 
-    def join_cells(cells):
-        cells = tuple(bytes(x) for x in cells)
-        return concat_cells(width, height, False, cells)
+        def join_cells(cells):
+            cells = tuple(bytes(x) for x in cells)
+            return concat_cells(width, height, False, cells)
 
-    def render_chr(ch):
-        if ch in box_chars:
-            cell = bytearray(len(space))
-            render_box_char(ch, cell, width, height)
-            return cell
-        return space
+        def render_chr(ch):
+            if ch in box_chars:
+                cell = bytearray(len(space))
+                render_box_char(ch, cell, width, height)
+                return cell
+            return space
 
-    pos = 0x2500
-    rows = []
-    space_row = join_cells(repeat(space, 32))
+        pos = 0x2500
+        rows = []
+        space_row = join_cells(repeat(space, 32))
 
-    try:
-        for r in range(10):
-            row = []
-            for i in range(16):
-                row.append(render_chr(chr(pos)))
-                row.append(space)
-                pos += 1
-            rows.append(join_cells(row))
-            rows.append(space_row)
-        rgb_data = b''.join(rows)
-        width *= 32
-        height *= len(rows)
-        assert len(rgb_data) == width * height * 4, '{} != {}'.format(len(rgb_data), width * height * 4)
-        display_bitmap(rgb_data, width, height)
-    finally:
-        set_send_sprite_to_gpu(None)
+        try:
+            for r in range(10):
+                row = []
+                for i in range(16):
+                    row.append(render_chr(chr(pos)))
+                    row.append(space)
+                    pos += 1
+                rows.append(join_cells(row))
+                rows.append(space_row)
+            rgb_data = b''.join(rows)
+            width *= 32
+            height *= len(rows)
+            assert len(rgb_data) == width * height * 4, '{} != {}'.format(len(rgb_data), width * height * 4)
+            display_bitmap(rgb_data, width, height)
+        finally:
+            set_send_sprite_to_gpu(None)