Fix issue with cross referencing figures and tables in epub (#1426)

It seems we were correctly resolving the references, and adding a link to it, but no anchor id anywhere as we do for HTML

This changes insert an anchor id inside an empty span in the caption part. This is a quick solution as the id should be probably added to the figure or table itself.

Author: cderv

NEWS.md

@@ -2,6 +2,8 @@
 
 - Fix an issue with CSL using hanging indent style in `gitbook()` (thanks, @pablobernabeu, #1422).
 
+- Fix cross referencing of figures and tables in `epub_book()` format by correctly adding an anchor id at the caption level (thanks, @muschellij2, #766, @tstratopoulos, @jasonmosborne, #1399, @N0rbert, rstudio/bookdown-demo#42).
+
 - Adapt an `epub_book()` internal command-line argument passed to Pandoc for changes from version 3.0 and above (#1425).
 
 - Fix an issue with Pandoc 2.19 not rendering math by default in `epub3` format (#1417).

R/ebook.R

@@ -122,7 +122,7 @@ resolve_refs_md = function(content, ref_table, to_md = output_md()) {
       if (grepl(m, content[i])) {
         id = ''; sep = ':'
         type = gsub('^([^:]+).*$', '\\1', j)
-        if (type %in% theorem_abbr) {
+        if (type %in% c(theorem_abbr, 'fig', 'tab')) {
           id = sprintf('<span id="%s"></span>', j)
           sep = ''
         }