VNC — Alt+Tab and Window Switching from Mac

Context: connecting to kelp (XFCE, display :1) via RealVNC Viewer on macOS (EPAM_HXQLYJV0X2). By default RealVNC captures all key combinations including Cmd+Tab and Alt+Tab, sending them to the remote session.

Fix: Stop RealVNC Capturing System Keys

RealVNC Preferences → Expert tab → set SendSpecialKeys = false.

Effect: Alt+Tab, Cmd+Tab etc. are now handled by macOS. Use Cmd+Tab to switch between Mac apps. The VNC session no longer receives these combos.

Side effect: Alt+Tab no longer cycles windows inside the kelp XFCE session. Need an alternative window-switching key that RealVNC will pass through.

Goal: Use ≤ and ≥ as Window Switchers in kelp

On Mac keyboard: Option+, produces ≤ (U+2264) and Option+. produces ≥ (U+2265). These combos pass through RealVNC to the X11 session.

Goal: make these cycle XFCE windows forward/backward, replacing Alt+Tab / Alt+Shift+Tab.

Key Technical Facts

From xev output on display :1:

• Option+, → keycode 184, keysym lessthanequal, state 0x80 (Mod5)

• Option+. → keycode 197, keysym greaterthanequal, state 0x80 (Mod5)

• Mod5 = ISO_Level3_Shift = keycode 92. It is pressed first (state 0x0), then the key arrives with state 0x80.

xdotool key alt+Tab (with DISPLAY=:1) works correctly from the terminal to cycle windows — confirmed manually.

Installed packages: xbindkeys, xdotool (both present on kelp).

Attempts — In Progress

Attempt 1: xfwm4 custom shortcuts via xfconf

DISPLAY=:1 xfconf-query -c xfce4-keyboard-shortcuts -p '/xfwm4/custom/<Mod5>lessthanequal' -s 'cycle_windows_key' --create -t string
DISPLAY=:1 xfconf-query -c xfce4-keyboard-shortcuts -p '/xfwm4/custom/<Mod5>greaterthanequal' -s 'cycle_reverse_windows_key' --create -t string
DISPLAY=:1 xfwm4 --replace &

Result: keys were grabbed (characters no longer appeared in applications) but window cycling did not trigger. Root cause unclear — possibly xfwm4 opened its tabwin popup which immediately grabbed the keyboard, or the shortcut format is not quite right for a level-3 keysym.

These entries were removed afterwards:

DISPLAY=:1 xfconf-query -c xfce4-keyboard-shortcuts -p '/xfwm4/custom/<Mod5>lessthanequal' --reset
DISPLAY=:1 xfconf-query -c xfce4-keyboard-shortcuts -p '/xfwm4/custom/<Mod5>greaterthanequal' --reset

Attempt 2: xbindkeys with keysym format

# ~/.xbindkeysrc
"DISPLAY=:1 xdotool key --clearmodifiers alt+Tab"
  Mod5+lessthanequal

"DISPLAY=:1 xdotool key --clearmodifiers alt+shift+Tab"
  Mod5+greaterthanequal

Result: xbindkeys did not grab the keys — characters still appeared. Tried several keysym name variants and numeric keycode formats; none grabbed successfully. Note: xbindkeys --key captures the ISO_Level3_Shift modifier keydown rather than the composed key, making it unreliable for detecting these combos.

This config remains in ~/.xbindkeysrc but xbindkeys is not running at session end.

Current State (end of 2026-06-10 session)

• xfwm4 shortcuts: standard Alt+Tab / Alt+Shift+Tab only — working normally

• xbindkeys: installed but not running; ~/.xbindkeysrc has the Mod5+lessthanequal config from attempt 2

• xdotool: installed, confirmed working for xdotool key alt+Tab

• RealVNC SendSpecialKeys = false — this is the desired permanent state; use Cmd+Tab on Mac to switch Mac apps

Next Steps to Try

1. Revisit xfwm4 shortcut approach — add a debug command (e.g. write to a file) to confirm whether the action fires at all, independent of xdotool.

2. Try sxhkd (Simple X HotKey Daemon) — uses a different key grab mechanism than xbindkeys and may handle level-3 keysyms better.

3. Use xcape to remap at keysym level before xbindkeys/xfwm4 sees the key.

4. Investigate whether xfwm4 tabwin opens briefly but closes immediately (test by adding cycle_raise = true or checking if focus briefly changes).

5. Try remapping the physical keycode 184/197 via xmodmap to a simpler keysym that xbindkeys can grab reliably, then bind that.