When dropping items from inventory you can hold Q to constantly drop items from a stack. But if you do the same thing while using your inventory within a container, chest/ horse chest/ ect it does not constantly drop items.
see images for examples.
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No developer has ever commented on the above ticket, someone simple asserted once that it was a bug and reported it as such, if that ticket was never made no one would question if it was intented behavior or not, until a developer addresses it it should not be assumed that it is unintended.
Notice that the above ticket says "Mojang Priority: Normal". If the developers have assigned a priority to a ticket, that means that it is unintended behavior and the developers plan on fixing it.
until a developer addresses it it should not be assumed that it is unintended.
But the opposite applies as well: Until it hasn't been officially confirmed as intended, it should not be assumed that it is intended.
If I recall correctly that behavior just started occurring suddenly without it being mentioned in the release notes.
Edit: Nevermind, appears this already existed in 1.7.10 and possibly earlier.
Holding Q to drop items quickly might not be intended in the first place, see MC-118107.