Mouse selection in the transcript spills into a docked plugin Pane when it crosses lines

With a plugin Pane docked beside the transcript (fullscreen terminal, placement: 'dock'), selecting transcript text with the mouse is not confined to the transcript. Once the selection moves to another row, it extends across the pane's columns and takes the pane's text with it.

Expected: the selection stays in the region where it started (transcript or pane).

Steps to reproduce

1. Run a plugin that opens a pane with $.ui.open({ id, focus: true }), in a fullscreen terminal wide enough for it to dock (110+ columns).
2. Drag the mouse over transcript text, crossing two or more rows.
3. The selection includes the pane's content on the following rows.

Why a plugin can't fix it

• No "non-selectable" prop on Box, Text or Pane.
• No hook or event on selection.
• $.ui.selection() only reads what was already selected.

Suggested fix (any of these)

• Confine the selection to the region where the drag began.
• Add a non-selectable prop for elements and panes.

Workarounds

• Rectangular selection in the terminal (⌥⌘+drag in iTerm2, ⌥+drag in Terminal.app).
• Use the inline placement (narrow window or not fullscreen).