Add option to arrange Tagbar window vertically

autoload/tagbar.vim

@@ -1767,8 +1767,14 @@ function! s:OpenWindow(flags) abort
     endif
 
     let s:window_opening = 1
-    let openpos = g:tagbar_left ? 'topleft vertical ' : 'botright vertical '
-    exe 'silent keepalt ' . openpos . g:tagbar_width . 'split ' . '__Tagbar__'
+    if g:tagbar_vertical == 0
+        let openpos = g:tagbar_left ? 'topleft vertical ' : 'botright vertical '
+        let width = g:tagbar_width
+    else
+        let openpos = g:tagbar_left ? 'leftabove ' : 'rightbelow '
+        let width = g:tagbar_vertical
+    endif
+    exe 'silent keepalt ' . openpos . width . 'split ' . '__Tagbar__'
     unlet s:window_opening
 
     call s:InitWindow(autoclose)
@@ -3050,12 +3056,6 @@ function! s:ShowInPreviewWin() abort
         return
     endif
 
-    call s:GotoFileWindow(taginfo.fileinfo, 1)
-    call s:mark_window()
-
-    " Check whether the preview window is already open and open it if not.
-    " This has to be done before the :psearch below so the window is relative
-    " to the Tagbar window.
     let pwin_open = 0
     for win in range(1, winnr('$'))
         if getwinvar(win, '&previewwindow')
@@ -3064,14 +3064,32 @@ function! s:ShowInPreviewWin() abort
         endif
     endfor
 
+    " We want the preview window to be relative to the file window in normal
+    " (horizontal) mode, and relative to the Tagbar window in vertical mode,
+    " to make the best use of space.
+    if g:tagbar_vertical == 0
+        call s:GotoFileWindow(taginfo.fileinfo, 1)
+        call s:mark_window()
+    endif
+
+    " Open the preview window if it is not already open. This has to be done
+    " explicitly before the :psearch below to better control its positioning.
     if !pwin_open
         silent execute
             \ g:tagbar_previewwin_pos . ' pedit ' . taginfo.fileinfo.fpath
+        if g:tagbar_vertical != 0
+            silent execute 'vertical resize ' . g:tagbar_width
+        endif
         " Remember that the preview window was opened by Tagbar so we can
         " safely close it by ourselves
         let s:pwin_by_tagbar = 1
     endif
 
+    if g:tagbar_vertical != 0
+        call s:GotoFileWindow(taginfo.fileinfo, 1)
+        call s:mark_window()
+    endif
+
     " Use psearch instead of pedit since pedit essentially reloads the file
     " and creates an empty undo entry. psearch has to be called from the file
     " window, and since we only want matches in the current file we disable