Background opacity changes background color

[JMans15]

Describe the bug The background color changes when opacity is not 1

To Reproduce Steps to reproduce the behavior:

1. Set the background color to "C"
2. Put the window on top of plain "C"
3. Reduce opacity
4. See that the rendered color is not "C"

Screenshots Here is a comparison between opacity = 1 and opacity = 0.8, no blur, on top of the same color as in the config so that it shouldn't change the color through the window. The window decoration has the exact same color and transparency

Environment details

kitty 0.29.2 created by Kovid Goyal
Linux archlinux 6.4.8-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 03 Aug 2023 16:02:01 +0000 x86_64
Arch Linux 6.4.8-arch1-1 (/dev/tty)

Running under: X11
Frozen: False
Paths:
  kitty: /usr/bin/kitty
  base dir: /usr/lib/kitty
  extensions dir: /usr/lib/kitty/kitty
  system shell: /usr/bin/zsh
Loaded config files:
  /home/mansj/.config/kitty/kitty.conf

Config options different from defaults:
background_opacity      0.8
editor                  nvim
enabled_layouts         ['splits', 'tall', 'fat']
font_family             FiraCode Nerd Font
font_size               13.0
initial_window_height   (720, 'px')
initial_window_width    (1280, 'px')
remember_window_size    False
symbol_map:
    U+23fb - U+23fe → Symbols Nerd Font Mono
    U+2665 - U+2665 → Symbols Nerd Font Mono
    U+26a1 - U+26a1 → Symbols Nerd Font Mono
    U+2b58 - U+2b58 → Symbols Nerd Font Mono
    U+e000 - U+e00a → Symbols Nerd Font Mono
    U+e0a0 - U+e0a3 → Symbols Nerd Font Mono
    U+e0b0 - U+e0d4 → Symbols Nerd Font Mono
    U+e200 - U+e2a9 → Symbols Nerd Font Mono
    U+e300 - U+e3e3 → Symbols Nerd Font Mono
    U+e5fa - U+e6aa → Symbols Nerd Font Mono
    U+e700 - U+e7c5 → Symbols Nerd Font Mono
    U+ea60 - U+ebeb → Symbols Nerd Font Mono
    U+f000 - U+f2e0 → Symbols Nerd Font Mono
    U+f300 - U+f32f → Symbols Nerd Font Mono
    U+f400 - U+f4a9 → Symbols Nerd Font Mono
    U+f500 - U+f8ff → Symbols Nerd Font Mono
    U+f0001 - U+f1af0 → Symbols Nerd Font Mono
tab_activity_symbol     •
tab_bar_edge            1
tab_bar_style           powerline
tab_title_template      {fmt.fg.orange}{bell_symbol}{activity_symbol}{fmt.fg.tab}{title}
window_padding_width    FloatEdges(left=5.0, top=5.0, right=5.0, bottom=5.0)
Added shortcuts:
    kitty_mod+alt+enter →  launch --location=hsplit
    kitty_mod+alt+left →  previous_tab
    kitty_mod+alt+right →  next_tab
Changed shortcuts:
    kitty_mod+down →  neighboring_window bottom
    kitty_mod+enter →  launch --location=vsplit
    kitty_mod+left →  neighboring_window left
    kitty_mod+right →  neighboring_window right
    kitty_mod+up →  neighboring_window up
Colors:
    active_border_color     #b4befe   
    active_tab_background   #cba6f7   
    active_tab_foreground   #11111b   
    background              #222638   
    bell_border_color       #f9e2af   
    color0                  #45475a   
    color1                  #f38ba8   
    color10                 #a6e3a1   
    color11                 #f9e2af   
    color12                 #89b4fa   
    color13                 #f5c2e7   
    color14                 #94e2d5   
    color15                 #a6adc8   
    color2                  #a6e3a1   
    color3                  #f9e2af   
    color4                  #89b4fa   
    color5                  #f5c2e7   
    color6                  #94e2d5   
    color7                  #bac2de   
    color8                  #585b70   
    color9                  #f38ba8   
    cursor                  #f5e0dc   
    cursor_text_color       #222638   
    foreground              #cdd6f4   
    inactive_border_color   #6c7086   
    inactive_tab_background #181825   
    inactive_tab_foreground #cdd6f4   
    mark1_background        #b4befe   
    mark1_foreground        #222638   
    mark2_background        #cba6f7   
    mark2_foreground        #222638   
    mark3_background        #74c7ec   
    mark3_foreground        #222638   
    selection_background    #f5e0dc   
    selection_foreground    #222638   
    tab_bar_background      #11111b   
    url_color               #f5e0dc   

Important environment variables seen by the kitty process:
    PATH                                /usr/local/sbin:/usr/local/bin:/usr/bin:/home/mansj/.local/share/flatpak/exports/bin:/var/lib/flatpak/exports/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/usr/lib/rustup/bin
    LANG                                fr_BE.UTF-8
    SHELL                               /usr/bin/zsh
    DISPLAY                             :0
    USER                                mansj
    XCURSOR_SIZE                        24
    LC_TIME                             fr_BE.UTF-8
    XDG_CONFIG_DIRS                     /home/mansj/.config/kdedefaults:/etc/xdg
    XDG_CURRENT_DESKTOP                 KDE
    XDG_DATA_DIRS                       /home/mansj/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share:/usr/local/share:/usr/share
    XDG_RUNTIME_DIR                     /run/user/1000
    XDG_SEAT                            seat0
    XDG_SEAT_PATH                       /org/freedesktop/DisplayManager/Seat0
    XDG_SESSION_CLASS                   user
    XDG_SESSION_DESKTOP                 KDE
    XDG_SESSION_ID                      2
    XDG_SESSION_PATH                    /org/freedesktop/DisplayManager/Session1
    XDG_SESSION_TYPE                    x11
    XDG_VTNR                            2

Additional context I went through related issues and PRs and found about issue #6250 and commit e1ce6d0, but I don't know if it's the same problem and if that's what said commit was supposed to fix

[kovidgoyal]

Not something I care about, there is enough complexity in rendering with semi-transparency already. Patches welcome.

[JMans15]

Just in case someone runs into the exact same problem, setting the background color to #1f2233 instead of #222638 yields the exact right color. Note that this corresponds to a 5 units shift in value (hue, sat, val) Also, I don't think it's a good reason to close an issue, someone else might want to look into it