vim-patch:8.2.4660: cursorcolumn is sometimes not correct

Problem:    Cursorcolumn is sometimes not correct.
Solution:   Recompute the cursor column when entering Insert mode and the
            cursor is on a character wider than a screen cell.
782c6744b4

src/nvim/edit.c

@@ -390,9 +390,13 @@ static void insert_enter(InsertState *s)
   trigger_modechanged();
   stop_insert_mode = false;
 
-  // Need to recompute the cursor position, it might move when the cursor is
-  // on a TAB or special character.
-  curs_columns(curwin, true);
+  // Need to recompute the cursor position, it might move when the cursor
+  // is on a TAB or special character.
+  // ptr2cells() treats a TAB character as double-width.
+  if (ptr2cells(get_cursor_pos_ptr()) > 1) {
+    curwin->w_valid &= ~VALID_VIRTCOL;
+    curs_columns(curwin, true);
+  }
 
   // Enable langmap or IME, indicated by 'iminsert'.
   // Note that IME may enabled/disabled without us noticing here, thus the

src/nvim/testdir/test_highlight.vim

@@ -597,6 +597,28 @@ func Test_cursorline_with_visualmode()
   call delete('Xtest_cursorline_with_visualmode')
 endfunc
 
+func Test_cursorcolumn_insert_on_tab()
+  CheckScreendump
+
+  let lines =<< trim END
+    call setline(1, ['123456789', "a\tb"])
+    set cursorcolumn
+    call cursor(2, 2)
+  END
+  call writefile(lines, 'Xcuc_insert_on_tab')
+
+  let buf = RunVimInTerminal('-S Xcuc_insert_on_tab', #{rows: 8})
+  call TermWait(buf)
+  call VerifyScreenDump(buf, 'Test_cursorcolumn_insert_on_tab_1', {})
+
+  call term_sendkeys(buf, 'i')
+  call TermWait(buf)
+  call VerifyScreenDump(buf, 'Test_cursorcolumn_insert_on_tab_2', {})
+
+  call StopVimInTerminal(buf)
+  call delete('Xcuc_insert_on_tab')
+endfunc
+
 func Test_cursorcolumn_callback()
   CheckScreendump
   CheckFeature timers