I am using PlantUML in Markdown for some sequence diagrams, and I would like to be able to group participants accordingly.
In my sequence diagrams, I make use of the alt grouping keyword to distinguish between different outcomes. However, when I use the box command to group participants, the box doesn't include the alt group inside its shading.
In the question here, the OP asks how to group participants in PlantUML. My question is different because I would like to find out how to get the box coloring to include the test-case, instead of wrapping around it.
Existing participants grouping effect
@startuml
box "Internal Service" #LightBlue
participant Bob
participant Alice
end box
participant Other
Bob -> Alice : hello
Alice -> Other : hello
@enduml
which yields 
Original source code credit for this example comes from this question, and credit goes to Jean-Marc Volle
My problem
I have attached a minimal reproducible example below. Notice how the grey shading, which indicates a group, does not shade through the alt test-cases? I would like to know how I can have the shading be included.
@startuml
actor pm
box "app"
participant A
participant B
end box
participant C
participant D
A --> B: message
alt Test A
A --> B: something
else Test B
B --> C: something else
end group
@enduml
which yields
