From 6a53897c179ef8a3b0384eb6592fbea99fe4b0e3 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Fri, 9 May 2025 07:39:06 +0530
Subject: [PATCH] change_font_size: allow multiplying/dividing the current font
 size in addition to incrementing it

Fixes #8616
---
 docs/changelog.rst            |  2 ++
 kitty/boss.py                 | 12 +++++++++++-
 kitty/options/definition.py   |  5 +++++
 kitty/options/utils.py        |  2 +-
 kitty/rc/set_font_size.py     |  4 ++--
 tools/cmd/at/set_font_size.go |  3 ++-
 6 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/docs/changelog.rst b/docs/changelog.rst
index a8480a5ba..48331405d 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -158,6 +158,8 @@ Detailed list of changes
 
 - Launch action: Allow using an env var that resolves to a full command-line as the program to launch (:pull:`8613`)
 
+- :ac:`change_font_size` allow multiplying/dividing the current font size in addition to incrementing it (:iss:`8616`)
+
 0.41.1 [2025-04-03]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/kitty/boss.py b/kitty/boss.py
index ef0b482df..250072e1a 100644
--- a/kitty/boss.py
+++ b/kitty/boss.py
@@ -1358,7 +1358,17 @@ class Boss:
                 new_size = get_options().font_size
             else:
                 if increment_operation:
-                    new_size += (1 if increment_operation == '+' else -1) * amt
+                    match increment_operation:
+                        case '+':
+                            new_size += amt
+                        case '-':
+                            new_size -= amt
+                        case '*':
+                            new_size *= amt
+                        case '/':
+                            new_size /= amt
+                        case _:
+                            pass  # no-op
                 else:
                     new_size = amt
                 new_size = max(MINIMUM_FONT_SIZE, min(new_size, get_options().font_size * 5))
diff --git a/kitty/options/definition.py b/kitty/options/definition.py
index fd8d26ff2..585d39425 100644
--- a/kitty/options/definition.py
+++ b/kitty/options/definition.py
@@ -4182,6 +4182,11 @@ To setup shortcuts for specific font sizes::
 To setup shortcuts to change only the current OS window's font size::
 
     map kitty_mod+f6 change_font_size current 10.0
+
+To setup shortcuts to multiply/divide the font size:
+
+    map kitty_mod+f6 change_font_size all *2.0
+    map kitty_mod+f6 change_font_size all /2.0
 ''')  # }}}
 
 
diff --git a/kitty/options/utils.py b/kitty/options/utils.py
index d9616480b..c674d1686 100644
--- a/kitty/options/utils.py
+++ b/kitty/options/utils.py
@@ -199,7 +199,7 @@ def parse_change_font_size(func: str, rest: str) -> tuple[str, tuple[bool, str |
     c_all = vals[0].lower() == 'all'
     sign: str | None = None
     amt = vals[1]
-    if amt[0] in '+-':
+    if amt[0] in '+-*/':
         sign = amt[0]
         amt = amt[1:]
     return func, (c_all, sign, float(amt.strip()))
diff --git a/kitty/rc/set_font_size.py b/kitty/rc/set_font_size.py
index f92a52ad1..2ac95808f 100644
--- a/kitty/rc/set_font_size.py
+++ b/kitty/rc/set_font_size.py
@@ -13,7 +13,7 @@ class SetFontSize(RemoteCommand):
     protocol_spec = __doc__ = '''
     size+/float: The new font size in pts (a positive number). If absent is assumed to be zero which means reset to default.
     all/bool: Boolean whether to change font size in the current window or all windows
-    increment_op/choices.+.-: The string ``+`` or ``-`` to interpret size as an increment
+    increment_op/choices.+.-.*./: The string ``+``, ``-``, ``*`` or ``/`` to interpret size as an increment
     '''
 
     short_desc = 'Set the font size in the active top-level OS window'
@@ -22,7 +22,7 @@ class SetFontSize(RemoteCommand):
         ' that in kitty all sub-windows in the same OS window'
         ' must have the same font size. A value of zero'
         ' resets the font size to default. Prefixing the value'
-        ' with a :code:`+` or :code:`-` increments the font size by the specified'
+        ' with a :code:`+`, :code:`-`, :code:`*` or :code:`/` changes the font size by the specified'
         ' amount. Use -- before using - to have it not mistaken for a option. For example:'
         ' kitten @ set-font-size -- -2'
     )
diff --git a/tools/cmd/at/set_font_size.go b/tools/cmd/at/set_font_size.go
index a307dc80f..16875c1ce 100644
--- a/tools/cmd/at/set_font_size.go
+++ b/tools/cmd/at/set_font_size.go
@@ -3,11 +3,12 @@
 package at
 
 import (
+	"bytes"
 	"strconv"
 )
 
 func parse_set_font_size(arg string, payload *set_font_size_json_type) error {
-	if len(arg) > 0 && (arg[0] == '+' || arg[0] == '-') {
+	if len(arg) > 0 && (bytes.IndexByte([]byte{'+', '-', '/', '*'}, arg[0]) > -1) {
 		payload.Increment_op = arg[:1]
 		arg = arg[1:]
 	}