Shift-Clicking Upgrades while in a Cell Workbench does not put them into the cell upgrade slots, if they can currently not be placed there (because they're already installed).
The upgrades are instead added to the filter. This is somewhat unergonomic and any upgrade card should not participate in being shift-clicked into the filter.

If a network tool is present, the shift-clicking also goes between inventory and network-tool and does not prioritize the cell upgrade slots.