vim-patch:9.1.1600: using diff anchors with hidden buffers fails silently (#35218)

Problem:  diff: using diff anchors with hidden buffers fails silently
Solution: Give specific error message for diff anchors when using hidden
          buffers (Yee Cheng Chin).

Diff anchors currently will fail to parse if a buffer used for diff'ing
is hidden. Previously it would just fail as the code assumes it would
not happen normally, but this is actually possible to do if `closeoff`
and `hideoff` are not set in diffopt. Git's default diff tool "vimdiff3"
also takes advantage of this.

This fix this properly would require the `{address}` parser to be
smarter about whether a particular address relies on window position or
not (e.g. the `'.` address requires an active window, but `'a` or `1234`
do not). Since hidden diff buffers seem relatively niche, just provide a
better error message / documentation for now. This could be improved
later if there's a demand for it.

related: vim/vim#17615
closes: vim/vim#17904

cad3b2421d

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

src/nvim/diff.c

@@ -2553,8 +2553,13 @@ static int parse_diffanchors(bool check_only, buf_T *buf, linenr_T *anchors, int
         break;
       }
     }
-    if (bufwin == NULL) {
-      return FAIL;  // should not really happen
+    if (bufwin == NULL && *dia != NUL) {
+      // The buffer is hidden. Currently this is not supported due to the
+      // edge cases of needing to decide if an address is window-specific
+      // or not. We could add more checks in the future so we can detect
+      // whether an address relies on curwin to make this more fleixble.
+      emsg(_(e_diff_anchors_with_hidden_windows));
+      return FAIL;
     }
   }