If the point of supporting Ukraine is to support the international order of respecting borders, then an absolutist interpretation would mean you stop at your border when repelling invaders.
On the other hand, that would certainly result in invaders loading up on personnel and materiel on their side of the border until they reached some critical mass for a re invasion.
A lot of people might not remember the first Gulf War where the international community defending Kuwait stopped at the Iraq border. I think it could be argued that was a mistake on multiple levels, even ignoring everything we know that came after.
It's simply not credible for a group of countries responsible for constant invasions of other countries to claim to be defending borders or supporting any sort of international law. The US at this very moment is occupying a larger percentage of Syria than Russia is of Ukraine.
What on earth are you talking about occupying Syria?
Edit: they're misconstruing the 32-country military coalition that's been trying to degrade Da'esh since 2014 as the US military by itself occupying sovereign territory.
Some may remember the breathless daily & weekly map updates on the news showing areas controlled by Da'esh changing. Might remember the coalition partnering with various groups of differing militancy & reliability. I think including us (the coalition) fucking over Iraqi Kurds...? I believe because Syria hated them? Or loved them?
So, y'know, absolutely nothing like Russia's completely unprovoked, unilateral decision to invade Ukraine because Putin was afraid of Ukraine getting too chummy with NATO countries, possibility even considering joining NATO.
then an absolutist interpretation would mean you stop at your border when repelling invaders
Amusingly, that was the interpretation I've heard some years ago over at reddit from poles malding about USSR marching to Berlin and kicking nazis out of their country in the process.
"The side that stays within its fortifications is beaten"
Napoleon Bonaparte
Not only do you need to strike at the enemy's territories and hold it to win, you need to threaten to keep it if you want to restore your original borders. Going to the peace table with enemy cities your pocket is a classic way to negotiate for your own land back. The more Russian land the Ukranians take, the more likely we will see a restoration of old borders.
Funnily enough you quote the dude that first planned to ally with Russia but get swept into the prototype of modern geopolitics and attacked it, not staying within his fortifications, and that led to him and his empire being utterly and completely crushed. Though unlike current followers of the evolution of the same geopolitical strategy, he at least didn't had ample historical precedence for this madness.
I would say an absolutist would be justified in crossing the border if they offer that land back at the end of hostilities in exchange for other concessions.
Even if Ukraine doesn't reclaim all of their land they could offer Russia kursk for some other equal amount of Ukrainian land in any peace talks.
Otherwise if Russia refuses Ukraine is justified in keeping that land.