A Sublime Text plugin for keeping viewport position stable on (un)folding

adjust_viewport_on_fold.py

from typing import Optional, Tuple
import sublime_plugin


class ViewEventListener(sublime_plugin.ViewEventListener):

    def on_text_command(self, command_name: str, args: Optional[dict]) -> Optional[Tuple[str, dict]]:
        if 'fold' in command_name:
            view = self.view
            self._old_layout = view.text_to_layout(view.sel()[0].begin())
            self._old_viewport = view.viewport_position()
        return None

    def on_post_text_command(self, command_name: str, args: Optional[dict]) -> None:
        if 'fold' in command_name:
            view = self.view
            _, y_after = view.text_to_layout(view.sel()[0].begin())
            y_shift = y_after - self._old_layout[1]
            if y_shift != 0:
                old_viewport_x, old_viewport_y = self._old_viewport
                new_y = old_viewport_y + y_shift
                view.set_viewport_position((old_viewport_x, new_y), animate=False)