composebox_typeahead: Fix topic completion broken for long stream names.

Previous value of `40` didn't account for full length of stream name
which results in topic completion not working for stream names bigger
than that.

web/src/composebox_typeahead.ts

@@ -43,6 +43,9 @@ import type {UserPillData} from "./user_pill.ts";
 import {user_settings} from "./user_settings.ts";
 import * as util from "./util.ts";
 
+/* Maximum channel name length + link syntax (#**>**) + some topic characters */
+const MAX_LOOKBACK_FOR_TYPEAHEAD_COMPLETION = 60 + 6 + 20;
+
 // **********************************
 // AN IMPORTANT NOTE ABOUT TYPEAHEADS
 // **********************************
@@ -423,7 +426,7 @@ export function tokenize_compose_str(s: string): string {
 
     // We limit how far back to scan to limit potential weird behavior
     // in very long messages, and simplify performance analysis.
-    let min_i = s.length - 40;
+    let min_i = s.length - MAX_LOOKBACK_FOR_TYPEAHEAD_COMPLETION;
     if (min_i < 0) {
         min_i = 0;
     }

web/tests/composebox_typeahead.test.cjs

@@ -2014,7 +2014,12 @@ test("tokenizing", () => {
 
     // The following cases are kinda judgment calls...
     // max scanning limit is 40 characters until chars like @, # , / are found
-    assert.equal(ct.tokenize_compose_str("foo @toomanycharactersistooridiculoustocomplete"), "");
+    assert.equal(
+        ct.tokenize_compose_str(
+            "foo @toomanycharactersistooridiculoustoautocompletethatitexceedsalllimits",
+        ),
+        "",
+    );
     assert.equal(ct.tokenize_compose_str("foo #bar@foo"), "#bar@foo");
 });