compose: Fix extra space being added by quote-and-reply.

The correct behavior here is that we want to ensure there is
whitespace in between the syntax being added and the content on either
side.  Our smart_insert logic handled this for the cases that were
common with inserting emoji (etc.), but didn't handle the more complex
cases with "quote and reply".

Fixes #11702.

frontend_tests/node_tests/compose_ui.js

@@ -106,6 +106,31 @@ run_test('smart_insert', () => {
     assert.equal(textbox.val(), ':octopus: abc :smile: :airplane: :heart: ');
     assert(textbox.focused);
 
+    // Test handling of spaces for ```quote
+    textbox = make_textbox('');
+    textbox.caret(0);
+    textbox.blur();
+    compose_ui.smart_insert(textbox, '```quote\nquoted message\n```\n');
+    assert.equal(textbox.insert_text, '```quote\nquoted message\n```\n');
+    assert.equal(textbox.val(), '```quote\nquoted message\n```\n');
+    assert(textbox.focused);
+
+    textbox = make_textbox('');
+    textbox.caret(0);
+    textbox.blur();
+    compose_ui.smart_insert(textbox, "[Quoting…]\n");
+    assert.equal(textbox.insert_text, '[Quoting…]\n');
+    assert.equal(textbox.val(), '[Quoting…]\n');
+    assert(textbox.focused);
+
+    textbox = make_textbox('abc');
+    textbox.caret(3);
+    textbox.blur();
+    compose_ui.smart_insert(textbox, " test with space");
+    assert.equal(textbox.insert_text, ' test with space ');
+    assert.equal(textbox.val(), 'abc test with space ');
+    assert(textbox.focused);
+
     // Note that we don't have any special logic for strings that are
     // already surrounded by spaces, since we are usually inserting things
     // like emojis and file links.

static/js/compose_ui.js

@@ -16,12 +16,18 @@ exports.smart_insert = function (textarea, syntax) {
     var after_str = textarea.val().slice(pos);
 
     if (pos > 0) {
-        if (!is_space(before_str.slice(-1))) {
+        // If there isn't space either at the end of the content
+        // before the insert or (unlikely) at the start of the syntax,
+        // add one.
+        if (!is_space(before_str.slice(-1)) && !is_space(syntax[0])) {
             syntax = ' ' + syntax;
         }
     }
 
-    if (!(after_str.length > 0 && is_space(after_str[0]))) {
+    // If there isn't whitespace either at the end of the syntax or the
+    // start of the content after the syntax, add one.
+    if (!(after_str.length > 0 && is_space(after_str[0]) ||
+          syntax.length > 0 && is_space(syntax.slice(-1)))) {
         syntax += ' ';
     }