Displayed "merged" view of multi-selection in the selection panel

[abey79]

We currently support multi-selection. When multiple items are selected, their respective state and properties are displayed one after the other in the Selection Panel.

I'm proposing to consider an alternative where, instead, a single, merged view would be shown. This view would include the union of all states and properties of all selected items, with controls supporting a "mixed" state. For a given property, if all selected items share the same value, the control would display it. Otherwise, the control would display a "mixed" state.

For example, tristate checkbox are sometimes used for that purpose:

Pros

• This makes multi-selection much more useful ("I want to set the same visible history for these 3 unrelated entities")
• This is arguably closer to common expectation (debatable).
• It introduce the notion of "union" of selected items, which is some time required in other area (e.g. display state of multiple instances of the same entity in the time panel)
• ... ?

Cons

• Complexity
• All the Selection Panel UI/widgets must be "tristate" (easy for checkbox, can get more complex for other features)
• ... ?

[abey79]

@Wumpf states:

it’s fairly common to have a mode for both: Show up to n elements side by side selected or (usually the default!) merge them into a “single multiselect” that allows you to edit all of them at once