Reuse inccommand preview window (fix #11529) (#12612)

* Reuse inccommand preview window

Currently, show_sub (inside ex_substitute) creates a new split on each
run for its existing buffer, and ex_substitute calls close_windows for
it. This functionality seems to relay in delayed operations on window
structures where the close event on the newest window is "cancelled" by
win_grid_alloc. But for multigrid, there is optimization in place in
win_grid_alloc which causes any (unnecessary?) allocations to be
skipped, and thus inccommand preview window is not preserved but closed
immediately. Alternative fix would be to remove said optimization, but
the whole "lets create a new split each time and trash the earlier
window" seems too wasteful.

Fix #11529

* Update failing test

The failing test sets inccommand=split and does `:%s/.`, but isn't
expecting to get any contents for the preview window, other than the
windows status line. Update the test to include the preview window
contents too.

2 files changed, 56 insertions, 16 deletions

diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 8a0f2e634a..519978f4fb 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -109,6 +109,8 @@ typedef struct {
 # include "ex_cmds.c.generated.h"
 #endif
 
+static int preview_bufnr = 0;
+
 /// ":ascii" and "ga" implementation
 void do_ascii(const exarg_T *const eap)
 {
@@ -3244,7 +3246,7 @@ static char_u *sub_parse_flags(char_u *cmd, subflags_T *subflags,
 /// @param do_buf_event If `true`, send buffer updates.
 /// @return buffer used for 'inccommand' preview
 static buf_T *do_sub(exarg_T *eap, proftime_T timeout,
-                     bool do_buf_event)
+                     bool do_buf_event, handle_T bufnr)
 {
   long i = 0;
   regmmatch_T regmatch;
@@ -4198,7 +4200,7 @@ skip:
       }
       curbuf->b_changed = save_b_changed;  // preserve 'modified' during preview
       preview_buf = show_sub(eap, old_cursor, &preview_lines,
-                             pre_hl_id, pre_src_id);
+                             pre_hl_id, pre_src_id, bufnr);
       if (subsize > 0) {
         extmark_clear(orig_buf, pre_src_id, eap->line1-1, 0,
                       kv_last(preview_lines.subresults).end.lnum-1, MAXCOL);
@@ -5575,14 +5577,31 @@ void ex_helpclose(exarg_T *eap)
   }
 }
 
+/// Tries to enter to an existing window of given buffer. If no existing buffer
+/// is found, creates a new split.
+///
+/// Returns OK/FAIL.
+int sub_preview_win(buf_T *preview_buf)
+{
+  if (preview_buf != NULL) {
+    FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
+      if (wp->w_buffer == preview_buf) {
+        win_enter(wp, false);
+
+        return OK;
+      }
+    }
+  }
+  return win_split((int)p_cwh, WSP_BOT);
+}
+
 /// Shows the effects of the :substitute command being typed ('inccommand').
 /// If inccommand=split, shows a preview window and later restores the layout.
 static buf_T *show_sub(exarg_T *eap, pos_T old_cusr,
-                       PreviewLines *preview_lines, int hl_id, int src_id)
+                       PreviewLines *preview_lines, int hl_id, int src_id,
+                       handle_T bufnr)
   FUNC_ATTR_NONNULL_ALL
 {
-  static handle_T bufnr = 0;  // special buffer, re-used on each visit
-
   win_T *save_curwin = curwin;
   cmdmod_T save_cmdmod = cmdmod;
   char_u *save_shm_p = vim_strsave(p_shm);
@@ -5600,9 +5619,9 @@ static buf_T *show_sub(exarg_T *eap, pos_T old_cusr,
 
   bool outside_curline = (eap->line1 != old_cusr.lnum
                           || eap->line2 != old_cusr.lnum);
-  bool split = outside_curline && (*p_icm != 'n');
+  bool preview = outside_curline && (*p_icm != 'n');
   if (preview_buf == curbuf) {  // Preview buffer cannot preview itself!
-    split = false;
+    preview = false;
     preview_buf = NULL;
   }
 
@@ -5620,11 +5639,10 @@ static buf_T *show_sub(exarg_T *eap, pos_T old_cusr,
   linenr_T highest_num_line = 0;
   int col_width = 0;
 
-  if (split && win_split((int)p_cwh, WSP_BOT) != FAIL) {
+  if (preview && sub_preview_win(preview_buf) != FAIL) {
     buf_open_scratch(preview_buf ? bufnr : 0, "[Preview]");
     buf_clear();
     preview_buf = curbuf;
-    bufnr = preview_buf->handle;
     curbuf->b_p_bl = false;
     curbuf->b_p_ma = true;
     curbuf->b_p_ul = -1;
@@ -5713,7 +5731,7 @@ static buf_T *show_sub(exarg_T *eap, pos_T old_cusr,
   win_enter(save_curwin, false);  // Return to original window
   update_topline();
 
-  // Update screen now. Must do this _before_ close_windows().
+  // Update screen now.
   int save_rd = RedrawingDisabled;
   RedrawingDisabled = 0;
   update_screen(SOME_VALID);
@@ -5727,6 +5745,17 @@ static buf_T *show_sub(exarg_T *eap, pos_T old_cusr,
   return preview_buf;
 }
 
+/// Closes any open windows for inccommand preview buffer.
+void close_preview_windows(void)
+{
+    block_autocmds();
+    buf_T *buf = preview_bufnr ? buflist_findnr(preview_bufnr) : NULL;
+    if (buf != NULL) {
+      close_windows(buf, false);
+    }
+    unblock_autocmds();
+}
+
 /// :substitute command
 ///
 /// If 'inccommand' is empty: calls do_sub().
@@ -5736,7 +5765,9 @@ void ex_substitute(exarg_T *eap)
 {
   bool preview = (State & CMDPREVIEW);
   if (*p_icm == NUL || !preview) {  // 'inccommand' is disabled
-    (void)do_sub(eap, profile_zero(), true);
+    close_preview_windows();
+    (void)do_sub(eap, profile_zero(), true, preview_bufnr);
+
     return;
   }
 
@@ -5760,9 +5791,14 @@ void ex_substitute(exarg_T *eap)
   // Don't show search highlighting during live substitution
   bool save_hls = p_hls;
   p_hls = false;
-  buf_T *preview_buf = do_sub(eap, profile_setlimit(p_rdt), false);
+  buf_T *preview_buf = do_sub(eap, profile_setlimit(p_rdt), false,
+                              preview_bufnr);
   p_hls = save_hls;
 
+  if (preview_buf != NULL) {
+    preview_bufnr = preview_buf->handle;
+  }
+
   if (save_changedtick != buf_get_changedtick(curbuf)) {
     // Undo invisibly. This also moves the cursor!
     if (!u_undo_and_forget(1)) { abort(); }
@@ -5772,10 +5808,7 @@ void ex_substitute(exarg_T *eap)
     curbuf->b_u_time_cur = save_b_u_time_cur;
     buf_set_changedtick(curbuf, save_changedtick);
   }
-  if (buf_valid(preview_buf)) {
-    // XXX: Must do this *after* u_undo_and_forget(), why?
-    close_windows(preview_buf, false);
-  }
+
   curbuf->b_p_ul = save_b_p_ul;
   curwin->w_p_cul = save_w_p_cul;   // Restore 'cursorline'
   curwin->w_p_cuc = save_w_p_cuc;   // Restore 'cursorcolumn'
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 1a836b7a98..93684e8606 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -450,6 +450,11 @@ static uint8_t *command_line_enter(int firstc, long count, int indent)
   ExpandCleanup(&s->xpc);
   ccline.xpc = NULL;
 
+  if (s->gotesc) {
+    // There might be a preview window open for inccommand. Close it.
+    close_preview_windows();
+  }
+
   if (s->did_incsearch) {
     if (s->gotesc) {
       curwin->w_cursor = s->save_cursor;
@@ -1958,8 +1963,10 @@ static int command_line_changed(CommandLineState *s)
     update_topline();
 
     redrawcmdline();
+
   } else if (State & CMDPREVIEW) {
     State = (State & ~CMDPREVIEW);
+    close_preview_windows();
     update_screen(SOME_VALID);  // Clear 'inccommand' preview.
   }