compose: Fix restoring big drafts freezes the app.

We are using `.val` to set compose box content which is very fast vs
`setFieldText` which is very slow due to it doing a lot of
forced repaints. The major downside of using `val` here is that
user will not able to perform `undo` operation on this which doesn't
seem something user would want to do here.

Note that this effects compose content restored from drafts,
scheduled messages and on reload.

web/src/compose_actions.ts

@@ -347,7 +347,13 @@ export function start(raw_opts: ComposeActionsStartOpts): void {
     }
 
     if (opts.content !== undefined) {
-        compose_ui.insert_and_scroll_into_view(opts.content, $("textarea#compose-textarea"), true);
+        const replace_all_without_undo_support = true;
+        compose_ui.insert_and_scroll_into_view(
+            opts.content,
+            $("textarea#compose-textarea"),
+            false,
+            replace_all_without_undo_support,
+        );
         $(".compose_control_button_container:has(.add-poll)").addClass("disabled-on-hover");
         // If we were provided with message content, we might need to
         // display that it's too long.

web/src/compose_ui.ts

@@ -114,8 +114,15 @@ export function insert_and_scroll_into_view(
     content: string,
     $textarea: JQuery<HTMLTextAreaElement>,
     replace_all = false,
+    replace_all_without_undo_support = false,
 ): void {
-    if (replace_all) {
+    if (replace_all_without_undo_support) {
+        // setFieldText is very slow and noticeable when inserting 10k+
+        // characters of text like from a drafted response,
+        // but we use it since we want to support `undo`. If we don't want
+        // to support `undo`, we can use a faster method.
+        $textarea.val(content);
+    } else if (replace_all) {
         setFieldText($textarea[0]!, content);
     } else {
         insertTextIntoField($textarea[0]!, content);

web/tests/compose_actions.test.js

@@ -143,10 +143,15 @@ test("start", ({override, override_rewire, mock_template}) => {
 
     let compose_defaults;
     override(narrow_state, "set_compose_defaults", () => compose_defaults);
-    override(compose_ui, "insert_and_scroll_into_view", (content, $textarea, replace_all) => {
-        $textarea.val(content);
-        assert.ok(replace_all);
-    });
+    override(
+        compose_ui,
+        "insert_and_scroll_into_view",
+        (content, $textarea, replace_all, replace_all_without_undo_support) => {
+            $textarea.val(content);
+            assert.ok(!replace_all);
+            assert.ok(replace_all_without_undo_support);
+        },
+    );
 
     // Start stream message
     compose_defaults = {