GTK4.22.4: Border chars break in popup/visual mode; buffer text renders fine. #21192

Steps to reproduce

vim9script
# ╔════════════════╗
# ║                ║
# ║     double     ║
# ║                ║
# ╚════════════════╝
popup_clear()
popup_create(repeat(['xxxxx'], 10), {
	border: [],
})

Expected behaviour

Every border char is rendered fine.

Version of Vim

9.2.1021

Environment

Fedora Linux 44 (Workstation Edition)

VIM - Vi IMproved 9.2
Included patches: 1-1021
Compiled by myn@fedora
Huge version with GTK4 GUI.

Logs and stack traces