Add chunksize to thumbnail requests.

src/_h5ai/private/conf/options.json

@@ -352,6 +352,7 @@
     - delay: number, delay in milliseconds after "dom-ready" before thumb-requesting starts
     - size: number, size in pixel of the generated thumbnails
     - exif: boolean, use included EXIF thumbs if possible
+    - chunksize: int, number of thumbs per request
     */
     "thumbnails": {
         "enabled": true,
@@ -360,7 +361,8 @@
         "doc": ["x-pdf", "x-ps"],
         "delay": 1,
         "size": 240,
-        "exif": false
+        "exif": false,
+        "chunksize": 20
     },
 
     /*

src/_h5ai/public/js/lib/ext/thumbnails.js

@@ -10,7 +10,8 @@ const settings = Object.assign({
     doc: ['x-pdf', 'x-ps'],
     delay: 1,
     size: 100,
-    exif: false
+    exif: false,
+    chunksize: 20
 }, allsettings.thumbnails);
 const landscapeRatio = 4 / 3;
 
@@ -71,7 +72,7 @@ const requestQueue = queue => {
         };
     });
 
-    server.request({
+    return server.request({
         action: 'get',
         thumbs
     }).then(json => {
@@ -81,14 +82,19 @@ const requestQueue = queue => {
     });
 };
 
+const breakAndRequestQueue = queue => {
+    const len = queue.length;
+    const chunksize = settings.chunksize;
+    let p = Promise.resolve();
+    for (let i = 0; i < len; i += chunksize) {
+        p = p.then(() => requestQueue(queue.slice(i, i + chunksize)));
+    }
+};
+
 const handleItems = items => {
     const queue = [];
-
     each(items, item => queueItem(queue, item));
-
-    if (queue.length) {
-        requestQueue(queue);
-    }
+    breakAndRequestQueue(queue);
 };
 
 const onViewChanged = added => {