bootstrap_typeahead: Fix compose typeahead overflowing when expanded.

When compose box is expanded, typeahead overlfows the top of the window.
We move the typeahead to the bottom of the screen and let
preventOverflow shift it into the visible area.

web/src/bootstrap_typeahead.ts

@@ -346,6 +346,27 @@ export class Typeahead<ItemType extends string | object> {
             maxWidth: "none",
             theme: "popover-menu",
             placement: this.dropup ? "top-start" : "bottom-start",
+            popperOptions: {
+                modifiers: [
+                    {
+                        // This will only work if there is enough space on the fallback placement, otherwise
+                        // `preventOverflow` will be used to position it in the visible space.
+                        name: "flip",
+                        options: {
+                            fallbackPlacements: ["top-start", "bottom-start"],
+                        },
+                    },
+                    {
+                        name: "preventOverflow",
+                        options: {
+                            // This seems required to prevent overflow, maybe because our placements are
+                            // not the usual top, bottom, left, right.
+                            // https://popper.js.org/docs/v2/modifiers/prevent-overflow/#altaxis
+                            altAxis: true,
+                        },
+                    },
+                ],
+            },
             interactive: true,
             appendTo: () => document.body,
             showOnCreate: true,

web/src/compose_ui.ts

@@ -10,6 +10,7 @@ import {
     wrapFieldSelection,
 } from "text-field-edit";
 
+import type {Typeahead} from "./bootstrap_typeahead";
 import * as bulleted_numbered_list_util from "./bulleted_numbered_list_util";
 import * as common from "./common";
 import {$t} from "./i18n";
@@ -58,8 +59,13 @@ type SelectedLinesSections = {
 export let compose_spinner_visible = false;
 export let shift_pressed = false; // true or false
 export let code_formatting_button_triggered = false; // true or false
+export let compose_textarea_typeahead: Typeahead<object> | undefined;
 let full_size_status = false; // true or false
 
+export function set_compose_textarea_typeahead(typeahead: Typeahead<object>): void {
+    compose_textarea_typeahead = typeahead;
+}
+
 export function set_code_formatting_button_triggered(value: boolean): void {
     code_formatting_button_triggered = value;
 }
@@ -67,6 +73,10 @@ export function set_code_formatting_button_triggered(value: boolean): void {
 // Some functions to handle the full size status explicitly
 export function set_full_size(is_full: boolean): void {
     full_size_status = is_full;
+    // Show typeahead at bottom of textarea on compose full size.
+    if (compose_textarea_typeahead) {
+        compose_textarea_typeahead.dropup = !is_full;
+    }
 }
 
 export function is_full_size(): boolean {

web/src/composebox_typeahead.js

@@ -1157,27 +1157,30 @@ export function initialize_compose_typeahead(selector) {
         $element: $(selector),
         type: "textarea",
     };
-    new Typeahead(bootstrap_typeahead_input, {
-        items: max_num_items,
-        dropup: true,
-        // Performance note: We have trivial matcher/sorters to do
-        // matching and sorting inside the `source` field to avoid
-        // O(n) behavior in the number of users in the organization
-        // inside the typeahead library.
-        source: get_sorted_filtered_items,
-        highlighter_html: content_highlighter_html,
-        matcher() {
-            return true;
-        },
-        sorter(items) {
-            return items;
-        },
-        updater: content_typeahead_selected,
-        stopAdvance: true, // Do not advance to the next field on a Tab or Enter
-        automated: compose_automated_selection,
-        trigger_selection: compose_trigger_selection,
-        header_html: get_header_html,
-    });
+
+    compose_ui.set_compose_textarea_typeahead(
+        new Typeahead(bootstrap_typeahead_input, {
+            items: max_num_items,
+            dropup: true,
+            // Performance note: We have trivial matcher/sorters to do
+            // matching and sorting inside the `source` field to avoid
+            // O(n) behavior in the number of users in the organization
+            // inside the typeahead library.
+            source: get_sorted_filtered_items,
+            highlighter_html: content_highlighter_html,
+            matcher() {
+                return true;
+            },
+            sorter(items) {
+                return items;
+            },
+            updater: content_typeahead_selected,
+            stopAdvance: true, // Do not advance to the next field on a Tab or Enter
+            automated: compose_automated_selection,
+            trigger_selection: compose_trigger_selection,
+            header_html: get_header_html,
+        }),
+    );
 }
 
 export function initialize({on_enter_send}) {

web/tests/composebox_typeahead.test.js

@@ -17,6 +17,7 @@ const compose_ui = mock_esm("../src/compose_ui", {
     },
     cursor_inside_code_block: () => false,
     set_code_formatting_button_triggered: noop,
+    set_compose_textarea_typeahead: noop,
 });
 const compose_validate = mock_esm("../src/compose_validate", {
     validate_message_length: () => true,