Desktop split view: plugin AbovePrompt band and side pane only render in the primary pane

We maintain a plugin used by our whole team (mods: AbovePrompt band, Pane, $.ui.status, toasts). Most of us work with two to four sessions in the desktop app's split view. Only the session in the primary (left) pane ever gets the plugin's band and side pane. In every other pane:

• no ui.render for AbovePrompt or Pane reaches the plugin;
• session.attach arrives without viewport;
• ui.open still returns isPlaced: true, so the plugin can't tell its pane will never show.

Only the status line, toasts and the SessionMode chip render there, and in a four-column split the status line shows only about 10 characters of the plugin's text.

From the app bundle this