vim-patch:8.2.4338: an error from an expression mapping messes up the display

Problem:    An error from an expression mapping messes up the display.
Solution:   When the expression results in an empty string return K_IGNORE.
            In cmdline mode redraw the command line. (closes vim/vim#9726)
74a0a5b26d

src/nvim/getchar.c

@@ -1953,6 +1953,7 @@ static int handle_mapping(int *keylenp, bool *timedout, int *mapdepth)
       const bool save_may_garbage_collect = may_garbage_collect;
       const int save_cursor_row = ui_current_row();
       const int save_cursor_col = ui_current_col();
+      const int prev_did_emsg = did_emsg;
 
       vgetc_busy = 0;
       may_garbage_collect = false;
@@ -1968,6 +1969,26 @@ static int handle_mapping(int *keylenp, bool *timedout, int *mapdepth)
       ui_cursor_goto(save_cursor_row, save_cursor_col);
       ui_flush();
 
+      // If an error was displayed and the expression returns an empty
+      // string, generate a <Nop> to allow for a redraw.
+      if (prev_did_emsg != did_emsg && (map_str == NULL || *map_str == NUL)) {
+        char_u buf[4];
+        xfree(map_str);
+        buf[0] = K_SPECIAL;
+        buf[1] = KS_EXTRA;
+        buf[2] = KE_IGNORE;
+        buf[3] = NUL;
+        map_str = vim_strsave(buf);
+        if (State & CMDLINE) {
+          // redraw the command below the error
+          msg_didout = true;
+          if (msg_row < cmdline_row) {
+            msg_row = cmdline_row;
+          }
+          redrawcmd();
+        }
+      }
+
       vgetc_busy = save_vgetc_busy;
       may_garbage_collect = save_may_garbage_collect;
     } else {

src/nvim/testdir/test_mapping.vim

@@ -483,6 +483,38 @@ func Test_expr_map_restore_cursor()
   call delete('XtestExprMap')
 endfunc
 
+func Test_expr_map_error()
+  CheckScreendump
+
+  let lines =<< trim END
+      func Func()
+        throw 'test'
+        return ''
+      endfunc
+
+      nnoremap <expr> <F2> Func()
+      cnoremap <expr> <F2> Func()
+
+      call test_override('ui_delay', 10)
+  END
+  call writefile(lines, 'XtestExprMap')
+  let buf = RunVimInTerminal('-S XtestExprMap', #{rows: 10})
+  call TermWait(buf)
+  call term_sendkeys(buf, "\<F2>")
+  call TermWait(buf)
+  call term_sendkeys(buf, "\<CR>")
+  call VerifyScreenDump(buf, 'Test_map_expr_2', {})
+
+  call term_sendkeys(buf, ":abc\<F2>")
+  call VerifyScreenDump(buf, 'Test_map_expr_3', {})
+  call term_sendkeys(buf, "\<Esc>0")
+  call VerifyScreenDump(buf, 'Test_map_expr_4', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('XtestExprMap')
+endfunc
+
 " Test for mapping errors
 func Test_map_error()
   call assert_fails('unmap', 'E474:')