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

doc/tagbar.txt

@@ -388,11 +388,29 @@ Default: 0
 By default the Tagbar window will be opened on the right-hand side of vim. Set
 this option to open it on the left instead.
 
+If |g:tagbar_vertical| is used then setting this variable will open the Tagbar
+window at the top, otherwise it will open at the bottom.
+
 Example:
 >
         let g:tagbar_left = 1
 <
 
+                                                           *g:tagbar_vertical*
+g:tagbar_vertical~
+Default: 0
+
+If this is set to a positive value then the Tagbar window will be opened at
+the top or bottom of the Vim window instead of at the side. This can be useful
+for monitors that have been rotated into a vertical position. The value of
+this variable will determine the number of lines to use for the Tagbar window.
+See |g:tagbar_left| for configuring the position of the window.
+
+Example:
+>
+        let g:tagbar_vertical = 30
+<
+
                                                               *g:tagbar_width*
 g:tagbar_width~
 Default: 40
@@ -604,7 +622,7 @@ Example:
 
                                                      *g:tagbar_previewwin_pos*
 g:tagbar_previewwin_pos~
-Default: topleft
+Default: "topleft", or "rightbelow vertical" if |g:tagbar_vertical| is set
 
 The position of the preview window. Valid values are the window splitting
 commands that are described starting from |:vertical|. Set it to an empty

plugin/tagbar.vim

@@ -48,6 +48,11 @@ function! s:init_var(var, value) abort
     endif
 endfunction
 
+if !exists('g:tagbar_vertical') || g:tagbar_vertical == 0
+    let s:previewwin_pos = 'topleft'
+else
+    let s:previewwin_pos = 'rightbelow vertical'
+endif
 let s:options = [
     \ ['autoclose', 0],
     \ ['autofocus', 0],
@@ -59,15 +64,17 @@ let s:options = [
     \ ['hide_nonpublic', 0],
     \ ['indent', 2],
     \ ['left', 0],
-    \ ['previewwin_pos', 'topleft'],
+    \ ['previewwin_pos', s:previewwin_pos],
     \ ['show_visibility', 1],
     \ ['show_linenumbers', 0],
     \ ['singleclick', 0],
     \ ['sort', 1],
     \ ['systemenc', &encoding],
+    \ ['vertical', 0],
     \ ['width', 40],
     \ ['zoomwidth', 1],
 \ ]
+unlet s:previewwin_pos
 
 for [opt, val] in s:options
     call s:init_var(opt, val)