A KOReader patch that draws a horizontal separator line below the top header area, matching the length of the bottom status bar separator line.

2-header-separator-line.lua

--[[
    Draws a horizontal separator line below the top header area,
    matching the length of the bottom status bar separator line.

    INSTALLATION:
      Place this in your "koreader/patches/" folder and restart KOReader.

    CONFIGURATION:
      Set "header_height" below to match the height of your header.
      Something like (your current top-margin value + 10) should work good.
      e.g., if your top-margin is 20, then set it to 30.
      But set whatever you seem ideal to you.

      LICENSE: AGPL-3.0
--]]

local Blitbuffer = require("ffi/blitbuffer")
local Size = require("ui/size")
local Device = require("device")
local Screen = Device.screen
local ReaderView = require("apps/reader/modules/readerview")

-- ===========================!!!!!!!!!!!!!!!=========================== --
local header_height = 30

-- Line appearance:
local separator_color     = Blitbuffer.COLOR_BLACK       -- COLOR_BLACK, COLOR_GRAY, etc.
local separator_thickness = Size.line.medium             -- Size.line.thin / .medium / .thick, or a number
-- ===========================!!!!!!!!!!!!!!!=========================== --

local _ReaderView_paintTo_orig = ReaderView.paintTo

ReaderView.paintTo = function(self, bb, x, y)
    _ReaderView_paintTo_orig(self, bb, x, y)
    if self.render_mode ~= nil then return end -- EPUBs/reflowable only, not PDF/CBZ

    local margin = Size.span.horizontal_default
    local line_y = y + header_height

    bb:paintRect(x + margin, line_y, Screen:getWidth() - margin * 2, separator_thickness, separator_color)
end