Swallowing: Child window not at same position as parent

[dkabus]

Hyprland version

Hyprland 0.51.0 built from branch  at commit 46174f78b374b6cea669c48880877a8bdcf7802f  (version: bump to 0.51.0).
Date: Wed Sep 10 12:41:05 2025
Tag: v0.51.0, commits: 6418
built against:
 aquamarine 0.9.4
 hyprlang 0.6.3
 hyprutils 0.8.4
 hyprcursor 0.1.13
 hyprgraphics 0.1.5

no flags were set

Regression

No.

Describe the bug

When a parent window (alacritty terminal) swallows a child window (zathura PDF viewer), the child window is placed at the end of the stack in the master layout, instead of staying at the same position as the parent window.

I am unsure if this has something to do with the master layout or with general windowing.

Relevant part of the config:

general {
    layout = master
}
master {
    slave_count_for_center_master = 0
    inherit_fullscreen = true
}
misc {
    enable_swallow = true
    swallow_regex = ^(Alacritty)$
}

Reproduction steps

0. Enable swallowing.
1. Open multiple windows (in master layout).
2. Open a terminal window (as not the last window in the stack):

3. Trigger swallowing by running a child process in that terminal that opens a child window (i.e. open a PDF).
4. The parent window (terminal) vanishes and the new child window (PDF viewer) pops up at the end of the stack:

6. When closing, the terminal window is also at the end of the stack:

System info and config

systeminfo.txt

(While I have the plugin hyprsplit installed, this bug also exists without that plugin installed.)

[dkabus]

Turns out, it was possible to fix it by setting new_on_active = after

master {
    slave_count_for_center_master = 0
    inherit_fullscreen = true
    new_on_active = after
}