scrollbar: add an ascii scrollbar extension

autoload/airline/extensions/scrollbar.vim

@@ -0,0 +1,36 @@
+" MIT License. Copyright (c) 2013-2021
+" vim: et ts=2 sts=2 sw=2 et
+
+scriptencoding utf-8
+
+function! airline#extensions#scrollbar#calculate() abort
+  if winwidth(0) > 200 && get(w:, 'airline_active', 0)
+    let overwrite = 0
+    if &encoding ==? 'utf-8' && !get(g:, 'airline_symbols_ascii', 0)
+      let [left, right, middle] = [ '|', '|', '█']
+      let overwrite = 1
+    else
+      let [left, right, middle] = [ '[', ']', '-']
+    endif
+    let spc = get(g:, 'airline_symbols.space', ' ')
+    let width = 20 " max width, plus one border and indicator
+    let perc = (line('.') + 0.0) / (line('$') + 0.0)
+    let before = float2nr(round(perc * width))
+    if before > 0 && line('.') == 1
+      let before = 0
+      let left = (overwrite ? '' : left)
+    endif
+    let after  = width - before
+    if (after <= 1 && line('.') == line('$'))
+      let after = 0
+      let right = (overwrite ? '' : right)
+    endif
+    return left . repeat(spc,  before) . middle . repeat(spc, (width-before)) . right
+  else
+    return ''
+  endif
+endfunction
+
+function! airline#extensions#scrollbar#init(ext) abort
+  call airline#parts#define_function('scrollbar', 'airline#extensions#scrollbar#calculate')
+endfunction