feat: configure half-page viewport scroll distance

src/ruby_config.rb

@@ -145,6 +145,7 @@ module Vamp
       @popup = PopupConfig.new
       @labels = {}
       @scroll_distance = 0
+      @scroll_half_page_lines = nil
       @highlight_trailing_whitespace = false
       @whitespace_characters = {}
       @whitespace_scope = "all"
@@ -229,6 +230,12 @@ module Vamp
       @scroll_distance = distance.nil? ? nil : diff_line_count!(distance, "scroll_distance", 0)
     end
 
+    # @param lines [Integer, nil] positive screen rows, or nil for half the viewport
+    # @return [Integer, nil]
+    def scroll_half_page_lines=(lines)
+      @scroll_half_page_lines = lines.nil? ? nil : diff_line_count!(lines, "scroll_half_page_lines", 1)
+    end
+
     # @param enabled [Boolean]
     # @return [Boolean]
     def highlight_trailing_whitespace=(enabled)
@@ -343,6 +350,7 @@ module Vamp
         "git_blame" => @git_blame,
         "git_gutter" => @git_gutter,
         "scroll_distance" => @scroll_distance,
+        "scroll_half_page_lines" => @scroll_half_page_lines,
         "highlight_trailing_whitespace" => @highlight_trailing_whitespace,
         "whitespace_characters" => @whitespace_characters,
         "whitespace_scope" => @whitespace_scope,