[Shell / Shell Panel]: Allow slotted displayMode: float Shell Panels to position absolutely

[macandcheese]

Description

Currently, when using Shell (without content-behind), there really is no use for the "float" displayMode value - it positions itself over a swim lane and has a "floating shadow" - but it is not positioned absolutely over main slotted content area.

This means that layouts with one "dock" and one "float" slotted Shell Panel cannot be achieved without content-behind, which is not always desirable.

Acceptance Criteria

In a Shell (without content-behind), the "float" value of displayMode behaves as an absolutely positioned element, and truly "floats" above the content. It should behave similarly to "float" when content-behind is set on Shell.

Relevant Info

While it seems we want to keep content-behind for the time being as it can be useful for certain mapping needs, adjusting the "non content-behind" Shell story in this way will allow more layouts to be achieved.

Which Component

Shell / Shell Panel

Example Use Case

Current with a "float" Shell Panel slotted in "panel-end":

Desired with a "float" Shell Panel slotted in "panel-end":

Priority impact

p3 - want for upcoming milestone

Esri team

Calcite (design)

[macandcheese]

+1 from UC customer

[macandcheese]

Related: #1932

[geospatialem]

Added additional acceptance criteria and context to the related issue. Closing in favor of the efforts of #1932.