vim-patch:9.1.0934: hard to view an existing buffer in the preview window (#31605)

Problem:  hard to view an existing buffer in the preview window
Solution: add the :pbuffer command (Yinzuo Jiang)

Similar as `:pedit` and `:buffer` command. `:pbuffer` edits buffer [N]
from the buffer list in the preview window.

`:pbuffer` can also open special buffer, for example terminal buffer.

closes: vim/vim#16222

https://github.com/vim/vim/commit/a2a2fe841ed2efdbb1f8055f752a3a4d0988ae9d

Cherry-pick Test_popup_and_previewwindow_dump() changes from patch
9.0.0625.
Cherry-pick Run_noroom_for_newwindow_test() changes from patches
8.2.0432 and 9.0.0363.

Co-authored-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>

10 files changed, 214 insertions, 35 deletions

diff --git a/runtime/doc/index.txt b/runtime/doc/index.txt
index d81d59c56f..0182123a12 100644
--- a/runtime/doc/index.txt
+++ b/runtime/doc/index.txt
@@ -1472,6 +1472,7 @@ tag		command		action ~
 |:ownsyntax|	:ow[nsyntax]	set new local syntax highlight for this window
 |:packadd|	:pa[ckadd]	add a plugin from 'packpath'
 |:packloadall|	:packl[oadall]	load all packages under 'packpath'
+|:pbuffer|	:pb[uffer]	edit buffer in the preview window
 |:pclose|	:pc[lose]	close preview window
 |:pedit|	:ped[it]	edit file in the preview window
 |:perl|		:pe[rl]		execute perl command
diff --git a/runtime/doc/windows.txt b/runtime/doc/windows.txt
index a6e5984c82..d3c58a28a4 100644
--- a/runtime/doc/windows.txt
+++ b/runtime/doc/windows.txt
@@ -978,6 +978,13 @@ CTRL-W g }						*CTRL-W_g}*
 		it.  Make the new Preview window (if required) N high.  If N is
 		not given, 'previewheight' is used.
 
+							*:pb* *:pbuffer*
+:[N]pb[uffer][!] [+cmd] [N]
+		Edit buffer [N] from the buffer list in the preview window.
+		If [N] is not given, the current buffer remains being edited.
+		See |:buffer-!| for [!].  This will also edit a buffer that is
+		not in the buffer list, without setting the 'buflisted' flag.
+
 							*:ped* *:pedit*
 :ped[it][!] [++opt] [+cmd] {file}
 		Edit {file} in the preview window.  The preview window is
diff --git a/src/nvim/ex_cmds.lua b/src/nvim/ex_cmds.lua
index c8574c805c..0cdc397e9c 100644
--- a/src/nvim/ex_cmds.lua
+++ b/src/nvim/ex_cmds.lua
@@ -1947,6 +1947,12 @@ M.cmds = {
     func = 'ex_packloadall',
   },
   {
+    command = 'pbuffer',
+    flags = bit.bor(BANG, RANGE, BUFNAME, BUFUNL, COUNT, EXTRA, CMDARG, TRLBAR),
+    addr_type = 'ADDR_BUFFERS',
+    func = 'ex_pbuffer',
+  },
+  {
     command = 'pclose',
     flags = bit.bor(BANG, TRLBAR),
     addr_type = 'ADDR_NONE',
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index b32a8b867f..052bf3b9f7 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -4502,6 +4502,12 @@ static void ex_bunload(exarg_T *eap)
 /// :[N]sbuffer [N]      to buffer N
 static void ex_buffer(exarg_T *eap)
 {
+  do_exbuffer(eap);
+}
+
+/// ":buffer" command and alike.
+static void do_exbuffer(exarg_T *eap)
+{
   if (*eap->arg) {
     eap->errmsg = ex_errmsg(e_trailing_arg, eap->arg);
   } else {
@@ -7002,14 +7008,35 @@ static void ex_ptag(exarg_T *eap)
 static void ex_pedit(exarg_T *eap)
 {
   win_T *curwin_save = curwin;
+  prepare_preview_window();
+
+  // Edit the file.
+  do_exedit(eap, NULL);
+
+  back_to_current_window(curwin_save);
+}
+
+/// ":pbuffer"
+static void ex_pbuffer(exarg_T *eap)
+{
+  win_T *curwin_save = curwin;
+  prepare_preview_window();
+
+  // Go to the buffer.
+  do_exbuffer(eap);
 
+  back_to_current_window(curwin_save);
+}
+
+static void prepare_preview_window(void)
+{
   // Open the preview window or popup and make it the current window.
   g_do_tagpreview = (int)p_pvh;
   prepare_tagpreview(true);
+}
 
-  // Edit the file.
-  do_exedit(eap, NULL);
-
+static void back_to_current_window(win_T *curwin_save)
+{
   if (curwin != curwin_save && win_valid(curwin_save)) {
     // Return cursor to where we were
     validate_cursor(curwin);
diff --git a/test/functional/ui/popupmenu_spec.lua b/test/functional/ui/popupmenu_spec.lua
index a4bf602e57..beb3ae385c 100644
--- a/test/functional/ui/popupmenu_spec.lua
+++ b/test/functional/ui/popupmenu_spec.lua
@@ -1542,6 +1542,81 @@ describe('builtin popupmenu', function()
     end)
 
     if not multigrid then
+      describe('popup and preview window do not overlap', function()
+        before_each(function()
+          screen:try_resize(53, 20)
+        end)
+
+        -- oldtest: Test_popup_and_previewwindow_dump_pedit()
+        it('with :pedit', function()
+          exec([[
+            set previewheight=9
+            silent! pedit
+            call setline(1, map(repeat(["ab"], 10), "v:val .. v:key"))
+            exec "norm! G\<C-E>\<C-E>"
+          ]])
+          feed('o')
+          n.poke_eventloop()
+          feed('<C-X><C-N>')
+          screen:expect([[
+            ab0                                                  |
+            ab1                                                  |
+            ab2                                                  |
+            ab3                                                  |
+            ab4                                                  |
+            ab5                                                  |
+            ab6                                                  |
+            ab7                                                  |
+            ab8                                                  |
+            {s:ab0            }{c: }{3:ew][+]                               }|
+            {n:ab1            }{c: }                                     |
+            {n:ab2            }{c: }                                     |
+            {n:ab3            }{c: }                                     |
+            {n:ab4            }{s: }                                     |
+            {n:ab5            }{s: }                                     |
+            {n:ab6            }{s: }                                     |
+            ab0^                                                  |
+            {1:~                                                    }|
+            {4:[No Name] [+]                                        }|
+            {2:-- Keyword Local completion (^N^P) }{5:match 1 of 10}     |
+          ]])
+        end)
+
+        -- oldtest: Test_popup_and_previewwindow_dump_pbuffer()
+        it('with :pbuffer', function()
+          exec([[
+            set previewheight=9
+            silent! pbuffer
+            call setline(1, map(repeat(["ab"], 10), "v:val .. v:key"))
+            exec "norm! G\<C-E>\<C-E>\<C-E>"
+          ]])
+          feed('o')
+          n.poke_eventloop()
+          feed('<C-X><C-N>')
+          screen:expect([[
+            ab0                                                  |
+            ab1                                                  |
+            ab2                                                  |
+            ab3                                                  |
+            ab4                                                  |
+            ab5                                                  |
+            ab6                                                  |
+            ab7                                                  |
+            ab8                                                  |
+            {s:ab0            }{c: }{3:ew][+]                               }|
+            {n:ab1            }{c: }                                     |
+            {n:ab2            }{c: }                                     |
+            {n:ab3            }{s: }                                     |
+            {n:ab4            }{s: }                                     |
+            {n:ab5            }{s: }                                     |
+            ab0^                                                  |
+            {1:~                                                    }|*2
+            {4:[No Name] [+]                                        }|
+            {2:-- Keyword Local completion (^N^P) }{5:match 1 of 10}     |
+          ]])
+        end)
+      end)
+
       -- oldtest: Test_pum_with_preview_win()
       it('preview window opened during completion', function()
         exec([[
diff --git a/test/old/testdir/test_popup.vim b/test/old/testdir/test_popup.vim
index 33e86678c8..a24e133ce6 100644
--- a/test/old/testdir/test_popup.vim
+++ b/test/old/testdir/test_popup.vim
@@ -748,17 +748,11 @@ func Test_popup_and_preview_autocommand()
   bw!
 endfunc
 
-func Test_popup_and_previewwindow_dump()
+func s:run_popup_and_previewwindow_dump(lines, dumpfile)
   CheckScreendump
   CheckFeature quickfix
 
-  let lines =<< trim END
-    set previewheight=9
-    silent! pedit
-    call setline(1, map(repeat(["ab"], 10), "v:val .. v:key"))
-    exec "norm! G\<C-E>\<C-E>"
-  END
-  call writefile(lines, 'Xscript')
+  call writefile(a:lines, 'Xscript', 'D')
   let buf = RunVimInTerminal('-S Xscript', {})
 
   " wait for the script to finish
@@ -768,11 +762,30 @@ func Test_popup_and_previewwindow_dump()
   call term_sendkeys(buf, "o")
   call TermWait(buf, 50)
   call term_sendkeys(buf, "\<C-X>\<C-N>")
-  call VerifyScreenDump(buf, 'Test_popup_and_previewwindow_01', {})
+  call VerifyScreenDump(buf, a:dumpfile, {})
 
   call term_sendkeys(buf, "\<Esc>u")
   call StopVimInTerminal(buf)
-  call delete('Xscript')
+endfunc
+
+func Test_popup_and_previewwindow_dump_pedit()
+  let lines =<< trim END
+    set previewheight=9
+    silent! pedit
+    call setline(1, map(repeat(["ab"], 10), "v:val .. v:key"))
+    exec "norm! G\<C-E>\<C-E>"
+  END
+  call s:run_popup_and_previewwindow_dump(lines, 'Test_popup_and_previewwindow_pedit')
+endfunc
+
+func Test_popup_and_previewwindow_dump_pbuffer()
+  let lines =<< trim END
+    set previewheight=9
+    silent! pbuffer
+    call setline(1, map(repeat(["ab"], 10), "v:val .. v:key"))
+    exec "norm! G\<C-E>\<C-E>\<C-E>"
+  END
+  call s:run_popup_and_previewwindow_dump(lines, 'Test_popup_and_previewwindow_pbuffer')
 endfunc
 
 func Test_balloon_split()
diff --git a/test/old/testdir/test_preview.vim b/test/old/testdir/test_preview.vim
index b7b908e761..d24b09b7d2 100644
--- a/test/old/testdir/test_preview.vim
+++ b/test/old/testdir/test_preview.vim
@@ -15,6 +15,20 @@ func Test_Psearch()
   bwipe
 endfunc
 
+func s:goto_preview_and_close()
+  " Go to the preview window
+  wincmd P
+  call assert_equal(1, &previewwindow)
+  call assert_equal('preview', win_gettype())
+
+  " Close preview window
+  wincmd z
+  call assert_equal(1, winnr('$'))
+  call assert_equal(0, &previewwindow)
+
+  call assert_fails('wincmd P', 'E441:')
+endfunc
+
 func Test_window_preview()
   CheckFeature quickfix
 
@@ -23,17 +37,36 @@ func Test_window_preview()
   call assert_equal(2, winnr('$'))
   call assert_equal(0, &previewwindow)
 
-  " Go to the preview window
-  wincmd P
-  call assert_equal(1, &previewwindow)
-  call assert_equal('preview', win_gettype())
+  call s:goto_preview_and_close()
+endfunc
 
-  " Close preview window
-  wincmd z
+func Test_window_preview_from_pbuffer()
+  CheckFeature quickfix
+
+  call writefile(['/* some C code */'], 'Xpreview.c', 'D')
+  edit Xpreview.c
+  const buf_num = bufnr('%')
+  enew
   call assert_equal(1, winnr('$'))
+  exe 'pbuffer ' .  buf_num
+  call assert_equal(2, winnr('$'))
   call assert_equal(0, &previewwindow)
 
-  call assert_fails('wincmd P', 'E441:')
+  call s:goto_preview_and_close()
+endfunc
+
+func Test_window_preview_terminal()
+  CheckFeature quickfix
+  " CheckFeature terminal
+
+  term " ++curwin
+  const buf_num = bufnr('$')
+  call assert_equal(1, winnr('$'))
+  exe 'pbuffer' . buf_num
+  call assert_equal(2, winnr('$'))
+  call assert_equal(0, &previewwindow)
+
+  call s:goto_preview_and_close()
 endfunc
 
 func Test_window_preview_from_help()
diff --git a/test/old/testdir/test_statusline.vim b/test/old/testdir/test_statusline.vim
index c8162ced07..c9f79dfef7 100644
--- a/test/old/testdir/test_statusline.vim
+++ b/test/old/testdir/test_statusline.vim
@@ -220,6 +220,10 @@ func Test_statusline()
   wincmd j
   call assert_match('^\[Preview\],PRV\s*$', s:get_statusline())
   pclose
+  pbuffer
+  wincmd j
+  call assert_match('^\[Preview\],PRV\s*$', s:get_statusline())
+  pclose
 
   " %y: Type of file in the buffer, e.g., "[vim]". See 'filetype'.
   " %Y: Type of file in the buffer, e.g., ",VIM". See 'filetype'.
diff --git a/test/old/testdir/test_window_cmd.vim b/test/old/testdir/test_window_cmd.vim
index e173aa1e73..343bc9fd83 100644
--- a/test/old/testdir/test_window_cmd.vim
+++ b/test/old/testdir/test_window_cmd.vim
@@ -1185,20 +1185,20 @@ func Run_noroom_for_newwindow_test(dir_arg)
   let dir = (a:dir_arg == 'v') ? 'vert ' : ''
 
   " Open as many windows as possible
-  for i in range(500)
+  while v:true
     try
       exe dir . 'new'
     catch /E36:/
       break
     endtry
-  endfor
+  endwhile
 
-  call writefile(['first', 'second', 'third'], 'Xfile1')
-  call writefile([], 'Xfile2')
-  call writefile([], 'Xfile3')
+  call writefile(['first', 'second', 'third'], 'Xnorfile1')
+  call writefile([], 'Xnorfile2')
+  call writefile([], 'Xnorfile3')
 
   " Argument list related commands
-  args Xfile1 Xfile2 Xfile3
+  args Xnorfile1 Xnorfile2 Xnorfile3
   next
   for cmd in ['sargument 2', 'snext', 'sprevious', 'sNext', 'srewind',
 			\ 'sfirst', 'slast']
@@ -1209,13 +1209,13 @@ func Run_noroom_for_newwindow_test(dir_arg)
   " Buffer related commands
   set modified
   hide enew
-  for cmd in ['sbuffer Xfile1', 'sbnext', 'sbprevious', 'sbNext', 'sbrewind',
+  for cmd in ['sbuffer Xnorfile1', 'sbnext', 'sbprevious', 'sbNext', 'sbrewind',
 		\ 'sbfirst', 'sblast', 'sball', 'sbmodified', 'sunhide']
     call assert_fails(dir .. cmd, 'E36:')
   endfor
 
   " Window related commands
-  for cmd in ['split', 'split Xfile2', 'new', 'new Xfile3', 'sview Xfile1',
+  for cmd in ['split', 'split Xnorfile2', 'new', 'new Xnorfile3', 'sview Xnorfile1',
 		\ 'sfind runtest.vim']
     call assert_fails(dir .. cmd, 'E36:')
   endfor
@@ -1238,7 +1238,8 @@ func Run_noroom_for_newwindow_test(dir_arg)
     call assert_fails(dir .. 'lopen', 'E36:')
 
     " Preview window
-    call assert_fails(dir .. 'pedit Xfile2', 'E36:')
+    call assert_fails(dir .. 'pedit Xnorfile2', 'E36:')
+    call assert_fails(dir .. 'pbuffer', 'E36:')
     call setline(1, 'abc')
     call assert_fails(dir .. 'psearch abc', 'E36:')
   endif
@@ -1246,15 +1247,15 @@ func Run_noroom_for_newwindow_test(dir_arg)
   " Window commands (CTRL-W ^ and CTRL-W f)
   if a:dir_arg == 'h'
     call assert_fails('call feedkeys("\<C-W>^", "xt")', 'E36:')
-    call setline(1, 'Xfile1')
+    call setline(1, 'Xnorfile1')
     call assert_fails('call feedkeys("gg\<C-W>f", "xt")', 'E36:')
   endif
   enew!
 
   " Tag commands (:stag, :stselect and :stjump)
   call writefile(["!_TAG_FILE_ENCODING\tutf-8\t//",
-        \ "second\tXfile1\t2",
-        \ "third\tXfile1\t3",],
+        \ "second\tXnorfile1\t2",
+        \ "third\tXnorfile1\t3",],
         \ 'Xtags')
   set tags=Xtags
   call assert_fails(dir .. 'stag second', 'E36:')
@@ -1276,9 +1277,9 @@ func Run_noroom_for_newwindow_test(dir_arg)
   endif
 
   %bwipe!
-  call delete('Xfile1')
-  call delete('Xfile2')
-  call delete('Xfile3')
+  call delete('Xnorfile1')
+  call delete('Xnorfile2')
+  call delete('Xnorfile3')
   only
 endfunc
 
diff --git a/test/old/testdir/test_winfixbuf.vim b/test/old/testdir/test_winfixbuf.vim
index cc8ff86fa1..f7986fdda3 100644
--- a/test/old/testdir/test_winfixbuf.vim
+++ b/test/old/testdir/test_winfixbuf.vim
@@ -2545,6 +2545,18 @@ func Test_pedit()
   call assert_equal(l:other, bufnr())
 endfunc
 
+" Allow :pbuffer because, unlike :buffer, it uses a separate window
+func Test_pbuffer()
+  call s:reset_all_buffers()
+
+  let l:other = s:make_buffer_pairs()
+
+  exe 'pbuffer ' . l:other
+
+  execute "normal \<C-w>w"
+  call assert_equal(l:other, bufnr())
+endfunc
+
 " Fail :pop but :pop! is allowed
 func Test_pop()
   call s:reset_all_buffers()