scriblib/figure: make figure hyperlinks show start of figure

... instead of putting the figure's caption at the top of the
browser window.

I can't decide whether I like the technique here, which is to use
JavaScript to move a figure caption's <a name=...> when the page is
loaded.  On the one hand, fixing up HTML via load-time JavaScript code
seems ugly compared to generating the right HTML in the first place.
On the other hand, it simplifies the generatation of a
backend-independent Scribble document (i.e., don't try to generate
different target locations for HTML and Latex), and HTML-specific
behavior seems properly handled at the HTML/JavaScript level.

original commit: 2b33b8d9668988b67f6b68b9eb012eb6d8f417d5

pkgs/scribble-pkgs/scribble-lib/scriblib/figure.js

@@ -0,0 +1,27 @@
+
+AddOnLoad(function () {
+    /* Lift figure targets to the start of the figure's blockquote,
+       so that clicking on a target reference shows the figure
+       content, instead of scrolling the figure caption to the
+       top of the page. */
+    var targets = document.getElementsByTagName("a");
+    for (var i = 0; i < targets.length; i++) {
+        var a = targets[i];
+        var n = a.attributes["x-target-lift"];
+        if (n) {
+            var s = n.value;
+            var p = a.parentNode;
+            while (p && (p.className != s)) {
+                p = p.parentNode;
+            }
+            if (p) {
+                var cs = p.children;
+                a.parentNode.removeChild(a);
+                if (cs.length > 0)
+                    p.insertBefore(a, cs[0]);
+                else
+                    p.appendChild(a);
+            }
+        }
+    }
+});

pkgs/scribble-pkgs/scribble-lib/scriblib/figure.rkt

@@ -47,6 +47,15 @@
 
 (define centertext-style (make-style "Centertext" figure-style-extras))
 
+;; See "figure.js":
+(define figure-target-style
+  (make-style #f
+              (list
+               (make-attributes '((x-target-lift . "Figure")))
+               (make-js-addition
+                (path->main-collects-relative
+                 (collection-file-path "figure.js" "scriblib"))))))
+
 (define (make-figure-ref c s)
   (element (style "FigureRef" (list* (command-extras (list s))
                                      figure-style-extras))
@@ -100,6 +109,7 @@
   (counter-target figures tag 
                   "Figure" 
                   (if continue? " (continued): " ": ")
+                  #:target-style figure-target-style
                   #:continue? continue?))
 
 (define (ref-proc initial)

pkgs/scribble-pkgs/scribble-lib/scriblib/private/counter.rkt

@@ -19,13 +19,14 @@
       `(,(string->symbol (format "counter-~a" kind)) ,tag)
       `(counter (,(counter-name counter) ,tag ,@kind))))
 
-(define (counter-target counter tag label 
+(define (counter-target counter tag label
+                        #:target-style [target-style #f]
                         #:continue? [continue? #f]
                         . content)
   (let ([content (decode-content content)])
     (define c
       (make-target-element
-       #f
+       target-style
        (list
         (make-collect-element
          #f