Add key to show tag in preview window

autoload/tagbar.vim

@@ -922,11 +922,12 @@ function! s:MapKeys() abort
                             \ <LeftRelease><C-o>:call <SID>CheckMouseClick()<CR>
 
     let maps = [
-        \ ['jump',      'JumpToTag(0)'],
-        \ ['preview',   'JumpToTag(1)'],
-        \ ['nexttag',   'GotoNextToplevelTag(1)'],
-        \ ['prevtag',   'GotoNextToplevelTag(-1)'],
-        \ ['showproto', 'ShowPrototype(0)'],
+        \ ['jump',       'JumpToTag(0)'],
+        \ ['preview',    'JumpToTag(1)'],
+        \ ['previewwin', 'ShowInPreviewWin()'],
+        \ ['nexttag',    'GotoNextToplevelTag(1)'],
+        \ ['prevtag',    'GotoNextToplevelTag(-1)'],
+        \ ['showproto',  'ShowPrototype(0)'],
         \
         \ ['openfold',      'OpenFold()'],
         \ ['closefold',     'CloseFold()'],
@@ -2797,6 +2798,7 @@ function! s:PrintHelp() abort
         silent  put ='\" ' . s:get_map_str('jump') . ': Jump to tag definition'
         silent  put ='\" ' . s:get_map_str('preview') . ': As above, but stay in'
         silent  put ='\"    Tagbar window'
+        silent  put ='\" ' . s:get_map_str('previewwin') . ': Show tag in preview window'
         silent  put ='\" ' . s:get_map_str('nexttag') . ': Go to next top-level tag'
         silent  put ='\" ' . s:get_map_str('prevtag') . ': Go to previous top-level tag'
         silent  put ='\" ' . s:get_map_str('showproto') . ': Display tag prototype'
@@ -2996,6 +2998,18 @@ function! s:JumpToTag(stay_in_tagbar) abort
     endif
 endfunction
 
+" s:ShowInPreviewWin() {{{2
+function! s:ShowInPreviewWin() abort
+    let taginfo = s:GetTagInfo(line('.'), 1)
+
+    if empty(taginfo) || !taginfo.isNormalTag()
+        return
+    endif
+
+    execute 'topleft pedit +' . taginfo.fields.line . ' ' .
+          \ s:known_files.getCurrent(0).fpath
+endfunction
+
 " s:ShowPrototype() {{{2
 function! s:ShowPrototype(short) abort
     let taginfo = s:GetTagInfo(line('.'), 1)

doc/tagbar.txt

@@ -318,6 +318,8 @@ The following mappings are valid in the Tagbar window:
                 Map option: tagbar_map_jump
 p             Jump to the tag under the cursor, but stay in the Tagbar window.
                 Map option: tagbar_map_preview
+P             Open the tag in a |preview-window|.
+                Map option: tagbar_map_previewwin
 <LeftMouse>   When on a fold icon, open or close the fold depending on the
               current state.
 <2-LeftMouse> Same as <CR>. See |g:tagbar_singleclick| if you want to use a

plugin/tagbar.vim

@@ -80,11 +80,12 @@ if !exists('g:tagbar_iconchars')
 endif
 
 let s:keymaps = [
-    \ ['jump',      '<CR>'],
-    \ ['preview',   'p'],
-    \ ['nexttag',   '<C-N>'],
-    \ ['prevtag',   '<C-P>'],
-    \ ['showproto', '<Space>'],
+    \ ['jump',       '<CR>'],
+    \ ['preview',    'p'],
+    \ ['previewwin', 'P'],
+    \ ['nexttag',    '<C-N>'],
+    \ ['prevtag',    '<C-P>'],
+    \ ['showproto',  '<Space>'],
     \
     \ ['openfold',      ['+', '<kPlus>', 'zo']],
     \ ['closefold',     ['-', '<kMinus>', 'zc']],