fix(api): open_win fire Buf* events when !enter && !noautocmd if entered early

Problem: if switch_win{_noblock} fails to restore the old curwin after WinNew
(e.g: it was closed), wp will become the new curwin, but win_set_buf enter
events would still be blocked if !enter && !noautocmd.

Solution: fire them, as we've actually entered the new window.

Note: there's a problem of switch_win{_noblock} failing to restore the old
curwin, leaving us in wp without triggering WinEnter/WinLeave, but this affects
all callers of switch_win{_noblock} anyways. (It's also not clear how WinLeave
can be called if the old curwin was closed already).

src/nvim/api/win_config.c

@@ -294,7 +294,7 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
     tp = win_find_tabpage(wp);
   }
   if (tp && buf != wp->w_buffer) {
-    const bool noautocmd = !enter || fconfig.noautocmd;
+    const bool noautocmd = curwin != wp || fconfig.noautocmd;
     win_set_buf(wp, buf, noautocmd, err);
     if (!noautocmd) {
       tp = win_find_tabpage(wp);

test/functional/api/window_spec.lua

@@ -1571,6 +1571,16 @@ describe('API/win', function()
       )
       eq(false, eval('fired'))
     end)
+
+    it('fires Buf* autocommands when `!enter` if window is entered via autocommands', function()
+      exec([[
+        autocmd WinNew * ++once only!
+        let fired = v:false
+        autocmd BufEnter * ++once let fired = v:true
+      ]])
+      api.nvim_open_win(api.nvim_create_buf(true, true), false, { split = 'left' })
+      eq(true, eval('fired'))
+    end)
   end)
 
   describe('set_config', function()