For the frightened condition, do transparent objects block line of sight?

There seems to be some discrepancy regarding the frightened conditions’s line of sight rules, since line of sight is not clearly defined. Perhaps intentionally so. Frightened’s disadvantage on ability checks is what I’m concerned with here.

A frightened creature has disadvantage on ability checks and attack rolls while the source of its fear is within line of sight.

Since you can’t cast spells through wall of force or semi-transparent objects, should the frightened condition follow similar rules? Does a piece of glass block line of sight? Can you draw a metaphorical line of sight through transparent barriers and objects?

We already know that simply not being able to see the creature does not negate the disadvantage. My question is whether or not being able to see through a barrier affects that disadvantage.