PiP-PersistedElement rendered in front of Pip-buttons (Hiding call hangup and GoTo room buttons)

[toger5]

For persisted/sticky/always-on-screen widgets (like element call) we use a persistedElement (A dom element outside the #matrixchat element) to render the app itself.

For calls there are two buttons:

• Go back to room (room the call belongs to).
• End call

those buttons are renderd as a <aside> and as a child of the #matrixchat element.
With the tree isolation (contain: strict) we use with compound this is not possible:

 - #matrixchat
   ...
   - "buttons for pip view" (end call, to room)
   ...
 - persistedElement
     ...
     - IFrame

The DOM order of persistedElement ABOVE #matrixchat enforces it to also be rendered above hiding the "buttons for pip view" (end call, to room). We need to refactor the PipContainer to create a dom structure that allows the required rendering order: "buttons above widget"

[t3chguy]

Regressed by matrix-org/matrix-react-sdk#12052