vim-patch:8.1.1136: decoding of mouse click escape sequence is not tested (neovim#35551)

Problem:    Decoding of mouse click escape sequence is not tested.
Solution:   Add a test for xterm and SGR using low-level input.  Make
            low-level input execution with feedkeys() work.

vim/vim@905dd90

Co-authored-by: Bram Moolenaar <[email protected]>

Author: janlazo

src/nvim/api/vim.c

@@ -332,7 +332,7 @@ void nvim_feedkeys(String keys, String mode, Boolean escape_ks)
     if (!dangerous) {
       ex_normal_busy++;
     }
-    exec_normal(true);
+    exec_normal(true, lowlevel);
     if (!dangerous) {
       ex_normal_busy--;
     }

src/nvim/ex_docmd.c

@@ -7072,21 +7072,26 @@ void exec_normal_cmd(char *cmd, int remap, bool silent)
 {
   // Stuff the argument into the typeahead buffer.
   ins_typebuf(cmd, remap, 0, true, silent);
-  exec_normal(false);
+  exec_normal(false, false);
 }
 
 /// Execute normal_cmd() until there is no typeahead left.
 ///
 /// @param was_typed whether or not something was typed
-void exec_normal(bool was_typed)
+/// @param use_vpeekc  true to use vpeekc() to check for available chars
+void exec_normal(bool was_typed, bool use_vpeekc)
 {
   oparg_T oa;
+  int c;
 
+  // When calling vpeekc() from feedkeys() it will return Ctrl_C when there
+  // is nothing to get, so also check for Ctrl_C.
   clear_oparg(&oa);
   finish_op = false;
   while ((!stuff_empty()
           || ((was_typed || !typebuf_typed())
-              && typebuf.tb_len > 0))
+              && typebuf.tb_len > 0)
+          || (use_vpeekc && (c = vpeekc()) != NUL && c != Ctrl_C))
          && !got_int) {
     update_topline_cursor();
     normal_cmd(&oa, true);      // execute a Normal mode cmd