Transformation.display_to_source is never called

When I'm looking at the code, the only call to display_to_source is made in BufferControl.mouse_handler. If mouse_support=False, display_to_source is never called. If mouse_support=True, display_to_source is only called for clicks.

Here is some demo code:

from prompt_toolkit import Application
from prompt_toolkit.buffer import Buffer
from prompt_toolkit.key_binding import KeyBindings
from prompt_toolkit.layout.containers import Window
from prompt_toolkit.layout.controls import BufferControl
from prompt_toolkit.layout.layout import Layout
from prompt_toolkit.layout.processors import Processor, Transformation


class MyProcessor(Processor):
    def apply_transformation(self, transformation_input):
        return Transformation(
            fragments=transformation_input.fragments,

            # This exits the application as expected.
            #source_to_display=exit,

            # This does nothing unless mouse support is enabled and the
            # BufferControl is clicked.
            display_to_source=exit,
        )

root_container = Window(BufferControl(
    buffer=Buffer(),
    input_processors=[MyProcessor()],
))

kb = KeyBindings()
@kb.add('c-q')
def exit_(event):
    event.app.exit()

app = Application(
    layout=Layout(root_container),
    key_bindings=kb,
    mouse_support=True,
)
app.run()

The application runs normally until you click on it.

Setting mouse_support=False turns display_to_source into dead code.

Uncommenting source_to_display=exit confirms that the Transformation object is used.

Is this a bug or am I misunderstanding how this is supposed to work?

Start by reading BufferControl.mouse_handler and the Transformation handling around display_to_source, then run the demo from the issue with mouse support both enabled and disabled. Trace when source_to_display and display_to_source are consumed. Done means the intended display-to-source behavior is established and covered by a regression test.