Greenstone.Walker
Adventurer
The rule about a line of effect implies that there is something physically travelling from caster to target.When you're casting Hold Person at someone you can see on the other side of a window, what is physically moving from you to the target for the glass to stop?
Or if looked at yet another way, you're not summoning the magic into you-the-caster here, you're summoning it to manifest and do something over there.
Same as with a Lightning Bolt - there's nothing physically travelling between me-the-caster and where I start the bolt, and so I should be able to start it at a point I can see through a window.
The rules don't say exactly what it is, just that there is something making a connection (using Crafword's phrasing) from the spellcaster to their target, something which total cover blocks.