Fix shared MarkdownViewer showing stale content when switching tabs

Tabs 1-3 (Spec, Prompt Plan, Notes) share a single MarkdownViewer widget.
Cache each tab's content and re-apply it on tab switch so users see the
correct content instead of whatever was last loaded.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: shuveb

src/mfbt/tui/screens/feature_detail.py

@@ -61,6 +61,7 @@ def __init__(
         self._sidebar_data: dict[str, Any] = {}
         self._active_tab = 1
         self._loaded_tabs: set[int] = set()
+        self._tab_content: dict[int, str] = {}
 
     def compose(self) -> ComposeResult:
         title = self._feature_data.get("title", "Feature Detail")
@@ -129,8 +130,10 @@ def _switch_tab(self, tab: int) -> None:
         impl_table.display = tab == 4
         conv_content.display = tab == 5
 
-        # Load content for the tab if not already loaded
-        if tab not in self._loaded_tabs:
+        # Tabs 1-3 share the MarkdownViewer, so re-apply cached content on switch
+        if tab in (1, 2, 3) and tab in self._tab_content:
+            viewer.content = self._tab_content[tab]
+        elif tab not in self._loaded_tabs:
             self._loaded_tabs.add(tab)
             self.run_worker(lambda: self._load_tab_content(tab), thread=True)
 
@@ -160,6 +163,7 @@ def _load_tab_content(self, tab: int) -> None:
                     detail = self._data_provider.fetch_feature_detail(self._feature_id)
                     text = detail.get("spec_text", "")
                 content = text if text else "*No spec available*"
+                self._tab_content[tab] = content
                 self.app.call_from_thread(setattr, viewer, "content", content)
 
             elif tab == 2:  # Prompt Plan
@@ -172,6 +176,7 @@ def _load_tab_content(self, tab: int) -> None:
                     detail = self._data_provider.fetch_feature_detail(self._feature_id)
                     text = detail.get("prompt_plan_text", "")
                 content = text if text else "*No prompt plan available*"
+                self._tab_content[tab] = content
                 self.app.call_from_thread(setattr, viewer, "content", content)
 
             elif tab == 3:  # Notes
@@ -181,6 +186,7 @@ def _load_tab_content(self, tab: int) -> None:
                     self._feature_data.get("notes", ""),
                 )
                 content = text if text else "*No notes available*"
+                self._tab_content[tab] = content
                 self.app.call_from_thread(setattr, viewer, "content", content)
 
             elif tab == 4:  # Implementations