stats.lua: refactor page 4 scrolling into function

This scrolling implementation will be used for other pages in future
commits.

The comment said it takes up 20 lines of the terminal, but in reality
it was 22 lines.

player/lua/stats.lua

@@ -1006,6 +1006,25 @@ local function eval_ass_formatting()
 end
 
 
+-- Composes the output with header and scrollable content
+-- Returns string of the finished page and the actually chosen offset
+--
+-- header   : table of the header where each entry is one line
+-- content  : table of the content where each entry is one line
+-- offset   : the desired scroll offset of the content from the first line at the top
+local function compose_page(header, content, offset)
+    -- up to 22 lines for the terminal - so that mpv can also print
+    -- the status line without scrolling, and up to 40 lines for libass
+    -- because it can put a big performance toll on libass to process
+    -- many lines which end up outside (below) the screen.
+    local max_content_lines = (o.use_ass and 40 or 22) - #header
+    -- in the terminal the scrolling should stop once the last line is visible
+    local max_offset = o.use_ass and #content or #content - max_content_lines + 1
+    local from = max(1, min((offset or 1), max_offset))
+    local to = min(#content, from + max_content_lines - 1)
+    return table.concat(header) .. table.concat(content, "", from, to), from
+end
+
 -- Returns an ASS string with "normal" stats
 local function default_stats()
     local stats = {}
@@ -1055,20 +1074,15 @@ local function keybinding_info(after_scroll)
     append(header, "", {prefix=format("%s: {\\fs%s}%s{\\fs%s}", page.desc,
            o.font_size * 0.66, "(hint: scroll with ↑↓)", o.font_size), nl="",
            indent=""})
+    header = {table.concat(header)}
 
     if not kbinfo_lines or not after_scroll then
         kbinfo_lines = get_kbinfo_lines()
     end
-    -- up to 20 lines for the terminal - so that mpv can also print
-    -- the status line without scrolling, and up to 40 lines for libass
-    -- because it can put a big performance toll on libass to process
-    -- many lines which end up outside (below) the screen.
-    local term = not o.use_ass
-    local nlines = #kbinfo_lines
-    page.offset = max(1, min((page.offset or 1), term and nlines - 20 or nlines))
-    local maxline = min(nlines, page.offset + (term and 20 or 40))
-    return table.concat(header) ..
-           table.concat(kbinfo_lines, "", page.offset, maxline)
+
+    local res = nil
+    res, page.offset = compose_page(header, kbinfo_lines, page.offset)
+    return res
 end
 
 local function perf_stats()