feat: configure cursor distance from viewport edges

src/ruby_config.rb

@@ -141,6 +141,7 @@ module Vamp
       @bar = BarConfig.new
       @popup = PopupConfig.new
       @labels = {}
+      @scroll_distance = 0
       @diff_context_lines = 10
       @diff_expand_lines = 15
       @diff_view_mode = "split"
@@ -214,6 +215,12 @@ module Vamp
       end
     end
 
+    # @param distance [Integer, nil] nonnegative cursor margin, or nil to center
+    # @return [Integer, nil]
+    def scroll_distance=(distance)
+      @scroll_distance = distance.nil? ? nil : diff_line_count!(distance, "scroll_distance", 0)
+    end
+
     # @param lines [Integer] nonnegative context lines around each diff change
     # @return [Integer]
     def diff_context_lines=(lines)
@@ -285,6 +292,7 @@ module Vamp
         "file_explorer_view" => @file_explorer_view, "clipboard_mode" => @clipboard_mode,
         "git_blame" => @git_blame,
         "git_gutter" => @git_gutter,
+        "scroll_distance" => @scroll_distance,
         "top_bar" => @bar.top, "bottom_bar" => @bar.bottom, "labels" => @labels,
         "popup" => { "command" => @popup.command, "search" => @popup.search },
         "diff_context_lines" => @diff_context_lines, "diff_expand_lines" => @diff_expand_lines,