Unit Selection Subgroups



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=Background= It is often helpful to be able to see your unit selection grouped by unit type, or other properties complete with aggregate hitpoints per group to understand their current health.

=Benefits=
 * Ability to quickly understand the composition of a selection's breakdown
 * Ability to monitor a unit or group's health when they are offscreen
 * Ability to better coordinate attacks by quickly selecting subgroups and issuing orders
 * Ability to split orders amongst individuals in the group without losing the selection

=Behaviour=
 * Cycle through subgroups with the Tab key
 * Cycle through various methods of grouping units with some key
 * Group in a hierarchy of various levels, possibly by control group, unit type, individual
 * Key-bindable "back" button that can go back to the original selection
 * Selection preview on subgroup mouseover
 * Shift+click subgroups to remove them from the selection
 * Target a subgroup by issuing orders on their icon as you would do with their actual unit

=Selection Subgroups=
 * Unit type
 * Unit role (engineer, combat, recon, etc)
 * Unit remaining hitpoints (0-20%, 20%-40%, etc)
 * Unit movement speed
 * Unit terrain movement type (air, land, water, etc)
 * Units on screen and off screen (selecting units of a type, and filter by "on screen" would be really useful)

=Existing Implementations=

Homeworld 2
The unit selections in Homeworld 2 were grouped by type, and were complete with a back button to restore the original selection. Units could also be focused on by holding the Alt key and clicking on their avatar in the selection screen, equivalent to Alt+clicking the unit.

Supreme Commander
Unit selection subgroups existed in Supreme Commander, but would only show an aggregated group of one unit type as the smallest component. It wasn't possible to target indvidual units, and aggregate hitpoints weren't shown.

Starcraft II
Apparently Starcraft II has selection subgroups and the ability to cycle through them with Tab.

=References=
 * Feature: UI: Unit selection subgroups w/back button
 * Selected Units: Selection Summary A La Homeworld 2