OP#10323: Do not clear the remark label before setting its text

The form outlives the page it is built into, so once that page's browser has
left, NiceGUI deletes its client and the form holds a deleted label. clear()
resolves the element's client and raised, failing every otconfig loaded after
the first visit to the page. set_text() goes through update(), which skips a
deleted element. A label has no children, so the clear() did nothing on a
live page either.

Author: randy-seng

OTAnalytics/plugin_ui/nicegui_gui/pages/remarks_form/container.py

@@ -30,7 +30,6 @@ def build(self) -> None:
     def load_remark(self) -> None:
         if not self._remark_label:
             return
-        self._remark_label.clear()
         if remark := self._viewmodel.get_remark():
             self._remark_label.set_text(remark)
         else:

tests/unit/OTAnalytics/plugin_ui/nicegui_gui/pages/remarks_form/__init__.py

@@ -0,0 +1,84 @@
+"""Tests the form once the browser that showed its page has gone away."""
+
+import asyncio
+import gc
+import weakref
+from dataclasses import dataclass
+from unittest.mock import Mock
+
+from nicegui import Client, context, ui
+from nicegui.testing import User
+
+from OTAnalytics.plugin_ui.nicegui_gui.pages.remarks_form.container import RemarkForm
+
+A_REMARK = "Counted at the northern approach"
+PAGE_WITH_FORM = "/with-form"
+PAGE_ELSEWHERE = "/elsewhere"
+COLLECTION_ATTEMPTS = 50
+COLLECTION_INTERVAL_SECONDS = 0.01
+
+
+class TestAfterThePageIsGone:
+    async def test_loading_a_remark_does_not_raise(self, user: User) -> None:
+        """The form outlives the page it was built into: it is created once and
+        rebuilt on every visit. Once that visit's browser has left, NiceGUI
+        deletes the client, and loading a remark reached for it and raised
+        `RuntimeError`, failing every otconfig loaded after the first page visit.
+
+        @bug by randy-seng
+        """
+        target = create_target(create_given())
+        client_ids: list[str] = []
+
+        @ui.page(PAGE_WITH_FORM)
+        def page_with_form() -> None:
+            target.build()
+            client_ids.append(context.client.id)
+
+        @ui.page(PAGE_ELSEWHERE)
+        def page_elsewhere() -> None:
+            ui.label("elsewhere")
+
+        await user.open(PAGE_WITH_FORM)
+        await leave_page(user, client_ids[0])
+
+        target.load_remark()
+
+
+async def leave_page(user: User, client_id: str) -> None:
+    """Navigate away and let NiceGUI delete the page's client, as it does once the
+    browser has left.
+
+    Elements hold their client only weakly, so they notice it is gone only once it
+    has been collected. Opening another page first makes the test user let go of
+    it; the client's outbox loop lets go once it notices the deletion.
+    """
+    await user.open(PAGE_ELSEWHERE)
+    client = Client.instances[client_id]
+    collected = weakref.ref(client)
+    client.delete()
+    del client
+    for _ in range(COLLECTION_ATTEMPTS):
+        await asyncio.sleep(COLLECTION_INTERVAL_SECONDS)
+        gc.collect()
+        if collected() is None:
+            return
+    raise AssertionError("The deleted client was never collected")
+
+
+@dataclass
+class Given:
+    resource_manager: Mock
+    view_model: Mock
+
+
+def create_given() -> Given:
+    resource_manager = Mock()
+    resource_manager.get.return_value = "label"
+    view_model = Mock()
+    view_model.get_remark.return_value = A_REMARK
+    return Given(resource_manager=resource_manager, view_model=view_model)
+
+
+def create_target(given: Given) -> RemarkForm:
+    return RemarkForm(given.resource_manager, given.view_model)