If *the state legislature of Pennsylvania* decides to select electors with a coin flip, then no, it's none of Texas' business.
But the state legislature of Pennsylvania did not make the changes in electoral law that are in question here. Consequently Pennsylvania violated a contract, and Texas (and 19 other states as well) has every right to seek redress for that contract violation.
You see, it's not "Pennsylvania's election." If they were voting for city council members or state offices or even their own US Senators and Representatives, that would be "Pennsylvania's election"; as much as we detest elected office holders like Nancy Pelosi, we don't argue that we can influence the electoral laws that continue to return her to office. But a vote for President is everyone's election; its distinct procedures are made clear in the Constitution, and Pennsylvania violated those procedures.
SCOTUS might have decided that the will of the Pennsylvania legislature was somehow implicit in the PA Supreme Court ruling, but they didn't decide that because they refused even to consider the case, and in that SCOTUS failed the country. By not even giving a hearing to a case in which they have original (hence sole) jurisdiction, they told 20 states that there is no forum to consider the grievance; "Texas and the other 19 states, if someone else violates the contract, there's nothing you can do, we won't even listen to your argument".
What do people do when they learn that they have no forum for resolution of grievance within a contract? They exit the contract.
Here's the problem: there is no contract involved here, and the case is not analogous to, or judicially remediable under the terms of contract law.
Instead, this is a matter of state procedural rules, and those are governed not by the Federal Constitution, but by state law.
Thus, it is the duty of state Attorneys General to bring suit (or not) to ensure that the duties of state officials were faithfully executed.
This is a state issue, like it or not, because the Constitution of the United States does not mandate procedures to be promulgated or enforced by the elected or appointed officials of each state. As long as the
procedures themselves are not in conflict with the Constitution, as to due process or other incorporated rights, the Supreme Court has no power of judicial review over them.
Look - in case there's any question (and I would assume there is none), I voted for Donald Trump and wanted him to win. The fact that dirty deeds were done (and they damn well were) is a miserable artifact of multi-generational, one-party power in highly populated urban districts. That problem will not be solved by Federal judicial intervention in an issue that needs to be addressed by cultural and social influence at the state and local level.
If you agree with me that Progressivism is a cancer on our body politic, that corruption is their ally, chaos their means, and control over your life their end, then you must be prepared to get involved personally, because that is how we will defeat the disease. Go to your town and school board meetings. Ask hard questions and prepare to be challenged. Know your reasons and your answers. And do not be afraid.
All real change comes from the bottom-up. Top-down is for collectivists and their elitist masters.
We can win. But we need to look to ourselves first, to fight.