copy/paste: Extract visible_range() to fix error.

When we tried to copy/paste multiple rows up to
and including the last row in our view, we'd have
a blueslip error when the `for` loop checked the
condition `rows.id(row) <= ...` after we had
called `row = rows.next_visible(row)` on the last
row.  Basically, `rows.id()` would complain
about a non-existent row.

Now we extract that code into `visible_range`, so
that our `while` loop can exit as soon as we found
the last row in the range.

static/js/copy_and_paste.js

@@ -57,13 +57,15 @@ how modern browsers deal with copy/paste.  Just test
 your changes carefully.
 */
 function construct_copy_div(div, start_id, end_id) {
-    const start_row = current_msg_list.get_row(start_id);
+    const copy_rows = rows.visible_range(start_id, end_id);
+
+    const start_row = copy_rows[0];
     const start_recipient_row = rows.get_message_recipient_row(start_row);
     const start_recipient_row_id = rows.id_for_recipient_row(start_recipient_row);
     let should_include_start_recipient_header = false;
-
     let last_recipient_row_id = start_recipient_row_id;
-    for (let row = start_row; rows.id(row) <= end_id; row = rows.next_visible(row)) {
+
+    for (const row of copy_rows) {
         const recipient_row_id = rows.id_for_recipient_row(rows.get_message_recipient_row(row));
         // if we found a message from another recipient,
         // it means that we have messages from several recipients,

static/js/rows.js

@@ -41,6 +41,30 @@ exports.last_visible = function () {
     return $('.focused_table .selectable_row').last();
 };
 
+exports.visible_range = function (start_id, end_id) {
+    /*
+        Get all visible rows between start_id
+        and end_in, being inclusive on both ends.
+    */
+
+    const rows = [];
+
+    let row = current_msg_list.get_row(start_id);
+    let msg_id = exports.id(row);
+
+    while (msg_id <= end_id) {
+        rows.push(row);
+
+        if (msg_id >= end_id) {
+            break;
+        }
+        row = exports.next_visible(row);
+        msg_id = exports.id(row);
+    }
+
+    return rows;
+};
+
 exports.is_draft_row = function (row) {
     return row.find('.restore-draft').length >= 1;
 };