Show popup when node name extends beyond drawer width

In cases where the node name extends beyond the width of the drawer
window (often the case with long file names or a deeply nested tree),
show a popup to help the user view the full node name.

Author: brandon1024

autoload/fern.vim

@@ -35,6 +35,7 @@ call s:Config.config(expand('<sfile>:p'), {
       \ 'disable_drawer_auto_winfixwidth': 0,
       \ 'disable_drawer_auto_resize': 0,
       \ 'disable_drawer_smart_quit': get(g:, 'disable_drawer_auto_quit', 0),
+      \ 'disable_drawer_hover_popup': 0,
       \ 'disable_drawer_auto_restore_focus': 0,
       \ 'default_hidden': 0,
       \ 'default_include': '',

autoload/fern/internal/drawer.vim

@@ -51,6 +51,7 @@ function! fern#internal#drawer#init() abort
   call fern#internal#drawer#auto_winfixwidth#init()
   call fern#internal#drawer#auto_restore_focus#init()
   call fern#internal#drawer#smart_quit#init()
+  call fern#internal#drawer#hover_popup#init()
   call fern#internal#drawer#resize()
   setlocal winfixwidth
 endfunction

autoload/fern/internal/drawer/hover_popup.vim

@@ -0,0 +1,63 @@
+function! fern#internal#drawer#hover_popup#init() abort
+  if g:fern#disable_drawer_hover_popup
+    return
+  endif
+
+  if !exists('*popup_create')
+    call fern#logger#warn('hover popup is not supported, popup_create()
+          \ does not exist. Disable this message
+          \ with g:fern#disable_drawer_hover_popup.')
+    return
+  endif
+
+  augroup fern_internal_drawer_hover_popup_init
+    autocmd! * <buffer>
+    autocmd CursorMoved <buffer> call s:cursor_moved_event()
+  augroup END
+endfunction
+
+function! fern#internal#drawer#hover_popup#calculate_node_char_offset(node) abort
+  " find line offset where label text begins
+  let line = getline('.')
+  let labelbegin = charidx(line, strridx(line, a:node.label))
+  let labelbegin = labelbegin < 0 ? 0 : labelbegin
+
+  let windowid = win_getid()
+
+  " get cursor position in drawer window (char- and byte-indexed)
+  let charpos = getcursorcharpos(windowid)
+  let pos = getcurpos(windowid)
+
+  " get cursor position relative to screen
+  let cursorpos = screenpos(windowid, pos[1], pos[2])
+
+  " calculate screen column where label text begins
+  return cursorpos['col'] - charpos[2] + labelbegin
+endfunction
+
+function! fern#internal#drawer#hover_popup#should_display_popup() abort
+  return len(getline('.')) >= winwidth(0)
+endfunction
+
+function! s:cursor_moved_event() abort
+  let helper = fern#helper#new()
+
+  if fern#internal#drawer#hover_popup#should_display_popup()
+    call s:show_popup(helper)
+  endif
+endfunction
+
+function! s:show_popup(helper) abort
+  let node = a:helper.sync.get_cursor_node()
+  if node is# v:null
+    return
+  endif
+
+  let label_offset = fern#internal#drawer#hover_popup#calculate_node_char_offset(node)
+  call popup_create(l:node.label, {
+        \ 'line': 'cursor',
+        \ 'col': label_offset + 1,
+        \ 'moved': 'any',
+        \})
+endfunction
+

doc/fern.txt

@@ -411,7 +411,15 @@ VARIABLE						*fern-variable*
 	of Neovim to avoid unwilling resize reported as #294
 	https://github.com/lambdalisue/fern.vim/issues/294
 
-	Default: 0 
+	Default: 0
+
+*g:fern#disable_drawer_hover_popup*
+	Set 1 to disable popups shown when the name of a node extends beyond
+	the width of the drawer.
+
+	Note that this feature is not supported in Neovim.
+
+	Default: 0
 
 *g:fern#disable_drawer_smart_quit*
 	Set 1 to disable smart quit behavior when there are only two buffer