composebox_typeahead: Rewrite code for handling modifier+enter.

We've iterated on this code incorrectly something like 3 times now, so
it's worth rewriting it with a lot of comments in a way that makes
sense.

The main actual functional change here is that modified key + enter
now is consistently the opposite of enter (in terms of whether to
provide a newline or send the message) in all cases.

Fixes #6489.

frontend_tests/node_tests/composebox_typeahead.js

@@ -603,6 +603,20 @@ global.people.add(deactivated_user);
     event.target.id = 'some_non_existing_id';
     $('form#send_message_form').keydown(event);
 
+
+    // Setup jquery functions used in new_message_content enter
+    // handler.
+    var range_length = 0;
+    $('#new_message_content').range = function () {
+        return {
+            length: range_length,
+            range: noop,
+            start: 0,
+            end: 0 + range_length,
+        };
+    };
+    $('#new_message_content').caret = noop;
+
     event.keyCode = 13;
     event.target.id = 'subject';
     $('form#send_message_form').keydown(event);
@@ -624,6 +638,12 @@ global.people.add(deactivated_user);
     event.ctrlKey = false;
     event.altKey = true;
     $('form#send_message_form').keydown(event);
+
+    // Cover case where there's a least one character there.
+    range_length = 2;
+    $('form#send_message_form').keydown(event);
+
+    event.altKey = false;
     event.metaKey = true;
     $('form#send_message_form').keydown(event);
     event.target.id = 'private_message_recipient';

static/js/composebox_typeahead.js

@@ -129,20 +129,60 @@ function handle_keydown(e) {
                 nextFocus = false;
             }
 
-            // Send the message on Ctrl/Cmd-Enter or if the user has configured enter to
-            // send and the Shift/Ctrl/Cmd/Alt keys are not pressed.
-            // Otherwise, make sure to insert a newline instead
             if (e.target.id === "new_message_content" && code === 13) {
-                if ((!page_params.enter_sends && (e.metaKey || e.ctrlKey)) ||
-                    (page_params.enter_sends && !(e.shiftKey || e.ctrlKey || e.metaKey || e.altKey))
-                ) {
+                var has_non_shift_modifier_key = e.ctrlKey || e.metaKey || e.altKey;
+                var has_modifier_key = e.shiftKey || has_non_shift_modifier_key;
+                var this_enter_sends;
+                if (page_params.enter_sends) {
+                    // With the enter_sends setting, we should send
+                    // the message unless the user was holding a
+                    // modifier key.
+                    this_enter_sends = !has_modifier_key;
+                } else {
+                    // If enter_sends is not enabled, just hitting
+                    // enter should add a newline, but with a modifier
+                    // key held down, we should send.
+                    this_enter_sends = has_modifier_key;
+                }
+                if (this_enter_sends) {
                     e.preventDefault();
                     if ($("#compose-send-button").attr('disabled') !== "disabled") {
                         $("#compose-send-button").attr('disabled', 'disabled');
                         compose.finish();
                     }
+                    return;
                 }
-                // Don't prevent default -- just let the enter key go in as usual.
+
+                // Since this enter doesn't send, we just want to do
+                // the browser's default behavior for the "enter" key.
+                // Letting the browser handle it works great if the
+                // key actually pressed was enter or shift-enter.
+
+                // But the default browser behavior for ctrl/alt/meta
+                // + enter is to do nothing, so we need to emulate
+                // the browser behavior for "enter" in those cases.
+                //
+                // We do this using caret and range from jquery-caret.
+                if (has_non_shift_modifier_key) {
+                    var textarea = $("#new_message_content");
+
+                    // To properly emulate browser "enter", if the
+                    // user had selected something in the compose box,
+                    // we need those characters to be cleared.
+                    var range = textarea.range();
+                    if (range.length > 0) {
+                        textarea.range(range.start, range.end).range('');
+                    }
+
+                    // Now add the newline, remembering to resize the
+                    // compose box if needed.
+                    textarea.caret("\n");
+                    compose_ui.autosize_textarea();
+                    e.preventDefault();
+                    return;
+                }
+
+                // Fall through to native browser behavior, otherwise.
             }
         }
     }