I've noticed when filling out larger bulk entries to my inventory (after buying a box, for example) that if you navigate away from that page, there's no prompt to prevent losing any text in those boxes. Because of this, a single errant click will lose any text already typed out.

Which entry are you referring to? The Card List box?

In this case the "Add Cards" > "From a Text List" box that's available on the Inventory, Tradelist, Wishlist, and deck pages.

Normally that input is provided for when you need to just paste some text in there, not necessarily type them by hand. Have you tried to use the one-by-one entry for adding them? It helps with auto completion as well, and should be quicker than using the text list input.