just use 16:9 it's 2018 bro

https://forums.linuxmint.com/viewtopic.php?t=204531

try the commands in the last post (run xrandr and replace DFP-0 with your output)

nvidia-settings --assign CurrentMetaMode="DFP-0: 1024x768 { ViewPortIn=1024x768, ViewPortOut=804x768+110+0 }"

if you need to figure out other resolutions it's documented here

http://download.nvidia.com/XFree86/Linux-x86/302.07/README/configtwinview.html

I tried all of this already too. Closest I could get to what I want was using ViewPortOut=1440x1080+240+0 to make my desktop in 4:3 with the black bars. That almost works, but when I go into TF2/CS:S all of the menu items and their clickable space gets misaligned (e.g. I have to put my cursor near the center of the screen to select the Scout image/class on the left side). None of the panning options (both "Panning" in the NVIDIA X Server Settings interface and the PanningTrackingArea option listed in the updated documentation for my driver version) did anything to help that, either.

Even if I could get that option to work, I'm not really interested in having that much wasted space on my primary monitor at all times, I just want my 4:3 aspect ratio to show up properly in-game while still making full use of my 16:9 monitor space in every other window. It's clearly possible with other games (CS1.6 in OP) so why can't I get it to work like that with Source Engine/TF2?

changing the resolution in source games doesn't actually change the desktop resolution in my experience

the trick is making a script that manually changes the resolution of your desktop when you open the game, and returns back to your normal desktop resolution when you close it
1.- figure out the xrandr name assigned to your screen by typing "xrandr" on a terminal (for example VGA-1 or LVDS-1)
2.- open hl2.sh in your Team Fortress 2 folder and right under #1/bin/bash create a line and put "xrandr --output LVDSS-1 --mode 1024x768" (replace LVDS-1 with your screen name)
3.- at the end of the file, between "done" and "exit", create a line saying xrandr --output LVDS-1 --mode 1920x1080 (again, replace with your actual screen name, and your native monitor resolution of course). This will execute when you exit the game.
this works on my laptop

Well after a whole afternoon of searching, I found a combination of these instructions & ViewPortOut to be the answer to my problems!

Basically I just replaced "xrandr --output [display] --mode [resolution]" with "nvidia-settings --assign CurrentMetaMode="_______________" and filled the blank space with the normal settings I use for my dual monitor setup (from /etc/X11/xorg.conf), edited my viewport settings to {viewportin=1920x1080, viewportout=1440x1080+240+0} to make a cramped 4:3 desktop (while still being recognized as 1920x1080@144hz), and then made it change back to the original viewport settings at the end of the file.

Obviously I'd still like the old flat panel scaling functionality back but this will do for the time being, I guess I can always make a separate global bind/shortcut for restoring my viewport settings if I ever need to tab out of the game to do something. Thank you very much for your help!

https://i.imgur.com/BmdbELi.jpg

And my hl2.sh:

#!/bin/bash

nvidia-settings --assign CurrentMetaMode="DVI-I-1: 1920x1080_144 +0+267 {viewportin=1920x1080, viewportout=1440x1080+240+0}, HDMI-0: nvidia-auto-select +1920+0 {rotation=left}"

# figure out the absolute path to the script being run a bit
# non-obvious, the ${0%/*} pulls the path out of $0, cd's into the
# specified directory, then uses $PWD to figure out where that
# directory lives - and all this in a subshell, so we don't affect
# $PWD

GAMEROOT=$(cd "${0%/*}" && echo $PWD)

#determine platform
UNAME=`uname`
if [ "$UNAME" == "Darwin" ]; then
   # Workaround OS X El Capitan 10.11 System Integrity Protection (SIP) which does not allow
   # DYLD_INSERT_LIBRARIES to be set for system processes.
   if [ "$STEAM_DYLD_INSERT_LIBRARIES" != "" ] && [ "$DYLD_INSERT_LIBRARIES" == "" ]; then
      export DYLD_INSERT_LIBRARIES="$STEAM_DYLD_INSERT_LIBRARIES"
   fi
   # prepend our lib path to LD_LIBRARY_PATH
   export DYLD_LIBRARY_PATH="${GAMEROOT}"/bin:$DYLD_LIBRARY_PATH
elif [ "$UNAME" == "Linux" ]; then
   # prepend our lib path to LD_LIBRARY_PATH
   export LD_LIBRARY_PATH="${GAMEROOT}"/bin:$LD_LIBRARY_PATH
fi

if [ -z $GAMEEXE ]; then
	if [ "$UNAME" == "Darwin" ]; then
		GAMEEXE=hl2_osx
	elif [ "$UNAME" == "Linux" ]; then
		GAMEEXE=hl2_linux
	fi
fi

ulimit -n 2048

# enable nVidia threaded optimizations
export __GL_THREADED_OPTIMIZATIONS=1

# and launch the game
cd "$GAMEROOT"

# Enable path match if we are running with loose files
if [ -f pathmatch.inf ]; then
	export ENABLE_PATHMATCH=1
fi

# Do the following for strace:
# 	GAME_DEBUGGER="strace -f -o strace.log"
# Do the following for tcmalloc
#   LD_PRELOAD=../src/thirdparty/gperftools-2.0/.libs/libtcmalloc_debug.so:$LD_PRELOAD

STATUS=42
while [ $STATUS -eq 42 ]; do
	if [ "${GAME_DEBUGGER}" == "gdb" ] || [ "${GAME_DEBUGGER}" == "cgdb" ]; then
		ARGSFILE=$(mktemp $USER.hl2.gdb.XXXX)
		echo b main > "$ARGSFILE"

		# Set the LD_PRELOAD varname in the debugger, and unset the global version. This makes it so that
		#   gameoverlayrenderer.so and the other preload objects aren't loaded in our debugger's process.
		echo set env LD_PRELOAD=$LD_PRELOAD >> "$ARGSFILE"
		echo show env LD_PRELOAD >> "$ARGSFILE"
		unset LD_PRELOAD

		echo run $@ >> "$ARGSFILE"
		echo show args >> "$ARGSFILE"
		${GAME_DEBUGGER} "${GAMEROOT}"/${GAMEEXE} -x "$ARGSFILE"
		rm "$ARGSFILE"
	else
		${GAME_DEBUGGER} "${GAMEROOT}"/${GAMEEXE} "$@"
	fi
	STATUS=$?
done
nvidia-settings --assign CurrentMetaMode="DVI-I-1: 1920x1080_144 +0+267 {viewportin=1920x1080, viewportout=1920x1080+0+0}, HDMI-0: nvidia-auto-select +1920+0 {rotation=left}"
exit $STATUS