diff --git a/src/nvim/quickfix.c b/src/nvim/quickfix.c
index f7eae6f825..5071741751 100644
--- a/src/nvim/quickfix.c
+++ b/src/nvim/quickfix.c
@@ -1019,6 +1019,13 @@ restofline:
       // global file names
       status = qf_parse_file_pfx(idx, fields, qfl, tail);
       if (status == QF_MULTISCAN) {
+        char *s = skipwhite(tail);
+        size_t new_linelen = strlen(s);
+        if (new_linelen >= linelen) {
+          return QF_IGNORE_LINE;
+        }
+        linebuf = s;
+        linelen = new_linelen;
         goto restofline;
       }
     }
@@ -1665,7 +1672,7 @@ static int qf_parse_dir_pfx(int idx, qffields_T *fields, qf_list_T *qfl)
 }
 
 /// Parse global file name error format prefixes (%O, %P and %Q).
-static int qf_parse_file_pfx(int idx, qffields_T *fields, qf_list_T *qfl, char *tail)
+static int qf_parse_file_pfx(int idx, qffields_T *fields, qf_list_T *qfl, const char *tail)
 {
   fields->valid = false;
   if (*fields->namebuf == NUL || os_path_exists(fields->namebuf)) {
@@ -1676,7 +1683,6 @@ static int qf_parse_file_pfx(int idx, qffields_T *fields, qf_list_T *qfl, char *
     }
     *fields->namebuf = NUL;
     if (tail && *tail) {
-      STRMOVE(IObuff, skipwhite(tail));
       qfl->qf_multiscan = true;
       return QF_MULTISCAN;
     }
diff --git a/test/old/testdir/test_quickfix.vim b/test/old/testdir/test_quickfix.vim
index 5c12e8592a..62940ce627 100644
--- a/test/old/testdir/test_quickfix.vim
+++ b/test/old/testdir/test_quickfix.vim
@@ -7028,4 +7028,26 @@ func Test_quickfix_longline_noeol()
   call assert_equal(['okay'], readfile("XDONE"))
 endfunc
 
+func Test_efm_overlongline()
+  let save_efm = &efm
+  " %r captures the tail.
+  set efm=%+O(%.%#)%r,%f:%l:%m
+
+  " First line is longer than IOSIZE (1025) so the parser puts it in
+  " growbuf; the %r tail then far exceeds IObuff.
+  let lines = ['(short)' .. repeat('x', 4000), 'Xfile:10:msg']
+  call writefile(lines, 'Xqferrlong', 'D')
+
+  " Must complete without hanging or crashing.
+  cgetfile Xqferrlong
+
+  " The well-formed line that follows is still parsed.
+  let well_formed = filter(getqflist(), 'v:val.lnum == 10')
+  call assert_equal(1, len(well_formed))
+  call assert_equal('msg', well_formed[0].text)
+
+  let &efm = save_efm
+  call setqflist([], 'f')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab