The filter (middle mouse klick) does not work in merged chests. Would it be possible to change that in future versions? Setting filters is sometimes so necessary und practical..

Merged chests are normal chests as far as game is concerned. Is it possible to assign filters to slots in Steel Chest (I haven't played in a while)?

Normally the game does not support filters in chests, but Picker adds this feature but not to steel chests. So merged chests seem to be another thing than normal chests.

Could you link me the mod in question?

https://mods.factorio.com/mod/PickerInventoryTools It's this one.

This mod only allows to set filters on entities which support filters. And since normal chests (and merged ones) don't support it (because base game just doesn't support it) I can't just change merged chests to support filters, sorry.