UPDATE
It was a buried window manager tweak setting that fixed it.
Window Manager Tweaks> "skip windows that have "skip pager" or "skip taskbar set"
Not sure when or how it happened, but qjackctl no longer shows up in the window switching menu when I alt-tab.
Anybody got any ideas where to start looking for a culprit?
Here's all the relevant info I can think of.
Cheers.
Qt: 5.15.7
QjackCtl: 0.5.0
OS: Manjaro Linux x86_64
Kernel: 6.0.0-rt11-30-nspa
Uptime: 10 days, 4 hours, 50 mins
Packages: 1764 (pacman), 4 (flatpak)
Shell: bash 5.1.16
Resolution: 1920x1080
DE: Xfce 4.18
WM: Xfwm4
WM Theme: Matcha-sea
Theme: Adwaita-dark [GTK2/3]
Icons: Papirus-Maia [GTK2], Adwaita [GTK3]
Terminal: xfce4-terminal
Terminal Font: Source Code Pro 10
CPU: AMD FX-8350 (8) @ 4.112GHz
GPU: NVIDIA GeForce GTX 750 Ti
Memory: 7512MiB / 15916MiB
xwininfo: Window id: 0x5c00006 "JACK Audio Connection Kit [(default)] Started."
Absolute upper-left X: 636
Absolute upper-left Y: 809
Relative upper-left X: 2
Relative upper-left Y: 28
Width: 435
Height: 102
Depth: 24
Visual: 0x27
Visual Class: TrueColor
Border width: 0
Class: InputOutput
Colormap: 0x5c00005 (not installed)
Bit Gravity State: NorthWestGravity
Window Gravity State: NorthWestGravity
Backing Store State: NotUseful
Save Under State: yes
Map State: IsViewable
Override Redirect State: no
Corners: +636+809 -849+809 -849-169 +636-169
-geometry 435x102+636+809