Extracted code into compose_fade.js.

The compose_fade has three public exports:

    set_focused_recipient
    unfade_messages
    update_faded_messages

All code was pulled directly from compose.js, except for the
one-line setter of set_focused_recipient.  The focused_recipients
variable that used to be in compose.js was moved to compose_fade.js,
hence the need for the setter.

(imported from commit 462ca5d0d0bd58612d0197f3734a8c78de8c6d30)

static/js/compose_fade.js

@@ -0,0 +1,71 @@
+var compose_fade = (function () {
+
+var exports = {};
+
+var focused_recipient;
+
+exports.set_focused_recipient = function (recipient) {
+    focused_recipient = recipient;
+};
+
+exports.unfade_messages = function (clear_state) {
+    if (focused_recipient === undefined) {
+        return;
+    }
+
+    rows.get_table(current_msg_list.table_name).find(".recipient_row, .message_row")
+                                               .removeClass("faded").addClass("unfaded");
+    if (clear_state === true) {
+        focused_recipient = undefined;
+    }
+    ui.update_floating_recipient_bar();
+};
+
+function _update_faded_messages() {
+    // See also update_faded_messages(), which just wraps this with a debounce.
+    if (focused_recipient === undefined) {
+        return;
+    }
+
+    if ((focused_recipient.type === "stream" && focused_recipient.subject === "") ||
+        (focused_recipient.type === "private" && focused_recipient.reply_to === "")) {
+        exports.unfade_messages();
+        return;
+    }
+
+    var i;
+    var all_elts = rows.get_table(current_msg_list.table_name).find(".recipient_row, .message_row");
+    var should_fade_message = false;
+    // Note: The below algorithm relies on the fact that all_elts is
+    // sorted as it would be displayed in the message view
+    for (i = 0; i < all_elts.length; i++) {
+        var elt = $(all_elts[i]);
+        if (elt.hasClass("recipient_row")) {
+            should_fade_message = !util.same_recipient(focused_recipient, current_msg_list.get(rows.id(elt)));
+        }
+
+        // Usually we are not actually switching up the classes here, so the hasClass()
+        // calls here will usually short circuit two function calls that are more expensive.
+        // So, while the hasClass() checks are semantically unnecessary, they should improve
+        // performance.  See trac #1633 for more context.
+        if (should_fade_message) {
+            if (!elt.hasClass("faded")) {
+                elt.removeClass("unfaded").addClass("faded");
+            }
+        } else {
+            if (!elt.hasClass("unfaded")) {
+                elt.removeClass("faded").addClass("unfaded");
+            }
+        }
+    }
+
+    ui.update_floating_recipient_bar();
+}
+
+// See trac #1633.  For fast typists, calls to _update_faded_messages can
+// cause typing sluggishness.
+exports.update_faded_messages = _.debounce(_update_faded_messages, 150);
+
+return exports;
+
+}());