Setting the window_margin_width removes borders for multiple windows

[JimmeeX]

Describe the bug
On multiple windows, the border does not appear if I set a window_margin_width to be greater than 0.

To Reproduce
Steps to reproduce the behavior:

1. In kitty.conf, set window_margin_width=5.0
2. Run kitty --config ~/.config/kitty/kitty.conf
3. Create multiple windows (cmd + enter)
4. Error as shown in 1st screenshot

Expected behavior
I would expect to see borders (like in the 2nd screenshot), except with some margins separating each window.

Screenshots
1st Screenshot (when window_margin_width=5.0)

2nd Screenshot (when window_margin_width=0)

Environment details
OS: This issue occurs on both my Arch Linux and Mac OS Big Sur (11.0)

kitty --debug-config outputs

Mac OS Big Sur (11.0)

kitty 0.19.1 created by Kovid Goyal
Darwin JimmeeXs-MacBook-Pro-16.local 20.1.0 Darwin Kernel Version 20.1.0: Thu Sep 24 20:22:06 PDT 2020; root:xnu-7195.40.89.100.3~1/RELEASE_X86_64 x86_64
ProductName:	macOS ProductVersion:	11.0 BuildVersion:	20A5384c
Loaded config files: /Users/jameslin/.config/kitty/kitty.conf

Config options different from defaults:
window_margin_width FloatEdges(left=5.0, top=5.0, right=5.0, bottom=5.0)

Arch Linux

kitty 0.19.1 created by Kovid Goyal
Linux jimmeex-arch 5.8.13-arch1-1 #1 SMP PREEMPT Thu, 01 Oct 2020 20:40:35 +0000 x86_64
Arch Linux \r (\l)
LSB_VERSION=1.4
DISTRIB_ID=Arch
DISTRIB_RELEASE=rolling
DISTRIB_DESCRIPTION="Arch Linux"
Loaded config files: /home/james/.config/kitty/kitty.conf
Running under: X11

Config options different from defaults:
window_margin_width FloatEdges(left=5.0, top=5.0, right=5.0, bottom=5.0)

Additional context
This started to become an issue when I upgraded kitty to version 0.19.0. Prior to that, I did not experience this issue.