"Republicans have hijacked the confirmation process and stolen the Supreme Court majority"

https://www.democracydocket.com/news-alerts/democrats-introduce-bill-to-expand-u-s-supreme-court/

Mitch McConnell certainly hijacked the nomination process in the case of Merrick Garland, but the conservatives have a three person majority on the Court. So how did the Republicans steal the majority?

Does winning presidential elections by the rules and exercising the powers explicitly granted constitute theft?

Wouldn't manipulating the size of the Court for purely purposes, while constitutionally allowable, be as close or closer to the concept of "stealing" the majority?

Or do the Democrats somehow have a legitimate claim to proper and perpetual ownership of the Supreme Court?