fix(api): win_set_config fires unnecessary autocmds

Problem: win_set_config should have the observable effect of moving an existing
window to another place, but instead fires autocommands as if a new window was
created and entered (and does not fire autocommands reflecting a "return" to the
original window).

Solution: do not fire win_enter-related autocommands when splitting the window,
but continue to fire them when entering the window that fills the new space when
moving a window to a different tabpage, as the new curwin changes.

Also, remove "++once" from the WinEnter autocmd in the other test, as omitting
it also crashed Nvim before this fix.

2 files changed, 41 insertions, 21 deletions

diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index bb1117b3fe..e53e13e2a3 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -505,7 +505,7 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
       win->w_pos_changed = true;
     }
 
-    int flags = win_split_flags(fconfig.split, parent == NULL);
+    int flags = win_split_flags(fconfig.split, parent == NULL) | WSP_NOENTER;
 
     if (parent == NULL) {
       if (!win_split_ins(0, flags, win, 0)) {
@@ -514,24 +514,13 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
         return;
       }
     } else {
-      win_execute_T args;
-
-      tabpage_T *tp = win_find_tabpage(parent);
-      if (!win_execute_before(&args, parent, tp)) {
-        // TODO(willothy): how should we handle this / what should the message be?
-        api_set_error(err, kErrorTypeException, "Failed to switch to tabpage %d", tp->handle);
-        win_execute_after(&args);
-        return;
-      }
-      // This should return the same ptr to `win`, but we check for
-      // NULL to detect errors.
-      win_T *res = win_split_ins(0, flags, win, 0);
-      win_execute_after(&args);
-      if (!res) {
-        // TODO(willothy): What should this error message say?
-        api_set_error(err, kErrorTypeException, "Failed to split window");
-        return;
-      }
+      switchwin_T switchwin;
+      // `parent` is valid in its tabpage, so switch_win should not fail.
+      const int result = switch_win(&switchwin, parent, win_find_tabpage(parent), true);
+      (void)result;
+      assert(result == OK);
+      win_split_ins(0, flags, win, 0);
+      restore_win(&switchwin, true);
     }
     if (HAS_KEY_X(config, width)) {
       win_setwidth_win(fconfig.width, win);
@@ -539,7 +528,6 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
     if (HAS_KEY_X(config, height)) {
       win_setheight_win(fconfig.height, win);
     }
-    redraw_later(win, UPD_NOT_VALID);
     return;
   } else {
     win_config_float(win, fconfig);
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index a812d502eb..3914090814 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -1668,7 +1668,7 @@ describe('API/win', function()
       command('split | tabnew')
       local w = api.nvim_get_current_win()
       local t = api.nvim_get_current_tabpage()
-      command('tabfirst | autocmd WinEnter * ++once quit')
+      command('tabfirst | autocmd WinEnter * quit')
       api.nvim_win_set_config(0, { win = w, split = 'left' })
       -- New tabpage is now the only one, as WinEnter closed the new curwin in the original.
       eq(t, api.nvim_get_current_tabpage())
@@ -1684,6 +1684,38 @@ describe('API/win', function()
         pcall_err(api.nvim_win_set_config, 0, { win = w, split = 'left' })
       )
     end)
+
+    it('expected autocmds when moving window to other tabpage', function()
+      local new_curwin = api.nvim_get_current_win()
+      command('split')
+      local win = api.nvim_get_current_win()
+      command('tabnew')
+      local parent = api.nvim_get_current_win()
+      exec([[
+        tabfirst
+        let result = []
+        autocmd WinEnter * let result += ["Enter", win_getid()]
+        autocmd WinLeave * let result += ["Leave", win_getid()]
+        autocmd WinNew * let result += ["New", win_getid()]
+      ]])
+      api.nvim_win_set_config(0, { win = parent, split = 'left' })
+      -- Shouldn't see WinNew, as we're not creating any new windows, just moving existing ones.
+      eq({ 'Leave', win, 'Enter', new_curwin }, eval('result'))
+    end)
+
+    it('no autocmds when moving window within same tabpage', function()
+      local parent = api.nvim_get_current_win()
+      exec([[
+        split
+        let result = []
+        autocmd WinEnter * let result += ["Enter", win_getid()]
+        autocmd WinLeave * let result += ["Leave", win_getid()]
+        autocmd WinNew * let result += ["New", win_getid()]
+      ]])
+      api.nvim_win_set_config(0, { win = parent, split = 'left' })
+      -- Shouldn't see any of those events, as we remain in the same window.
+      eq({}, eval('result'))
+    end)
   end)
 
   describe('get_config', function()