sublime text script to create two new expand commands: to end of file and expand to section. a section is defined by a header using the Rstudio syntax '# SECTION TITLE ----------' or anything with 4 hashes '####'. flexible with comment sections as well.

sublime-section-expands.py

import sublime, sublime_plugin

class ExpandSelectionToEofCommand(sublime_plugin.TextCommand):
    def run(self, edit):
        v = self.view
        s = v.sel()
        eof = v.size()

        first_point = v.line(s[0]).a
        region_to_eof = sublime.Region(first_point, eof)
        s.clear()
        s.add(region_to_eof)

class ExpandSelectionToSectionCommand(sublime_plugin.TextCommand):
    def run(self, edit):

        v = self.view
        s = v.sel()

        def get_text(row, col):
            return v.substr(v.line(v.text_point(row, col)))

        def find_top(line, pattern):
            # print('%d' % line)
            if pattern.match(get_text(line, 0)): # match text
                top = v.line(v.text_point(line + 1, 0))
                # print('section values: (%d, %d)' % (top.a, top.b))
                return top
            else:
                return find_top(line - 1, pattern)

        def find_bottom(line, eof_line_num, pattern):
            # print('%d' % line)
            if pattern.match(get_text(line, 0)): # match text
                bottom = v.line(v.text_point(line - 1, 0))
                # print('section values: (%d, %d)' % (bottom.a, bottom.b))
                return bottom
            elif line == eof_line_num:
                bottom = v.line(v.text_point(eof_line_num, 0))
                # print('eof values: (%d, %d)' % (bottom.a, bottom.b))
                return bottom
            else:
                return find_bottom(line + 1, eof_line_num, pattern)

        # find pattern
        import re

        pattern_str = "^[ ]*# [#]{4,}|^#+[ ]+#+[ ][\w|\s]+[-]+$|^[ ]*[#]{4,}|^[ ]*#+ [\w|\s]+[-]+$|^`{3}"
        pattern = re.compile(pattern_str)

        # get current point and then look forward
        first_point = v.line(s[0]).a
        section_line = v.find(pattern_str, first_point)

        # if at section_line header, jump forward 1 spot then expand and reassign `first_point`
        # print('%d %d' % (first_point, section_line.a))
        if first_point == section_line.a:
            line = v.rowcol(section_line.a)[0] # get current line number
            first_point = v.line(v.text_point(line + 1, 0)).a

        # print('%d' % (first_point))
        # search for closest section top and bottom
        current_line = v.rowcol(first_point)[0] # get line number
        eof = v.size()
        eof_line_num = v.rowcol(eof)[0]
        top_line = find_top(current_line, pattern)
        bottom_line = find_bottom(current_line, eof_line_num, pattern)
        section = sublime.Region(top_line.a, bottom_line.b)



        s.clear()
        s.add(section)