SystemMonitor: Don't generate backtraces while not looking at them

Userland/Applications/SystemMonitor/ThreadStackWidget.cpp

@@ -37,21 +37,30 @@ ThreadStackWidget::ThreadStackWidget()
     layout()->set_margins({ 4, 4, 4, 4 });
     m_stack_editor = add<GUI::TextEditor>();
     m_stack_editor->set_mode(GUI::TextEditor::ReadOnly);
-
-    m_timer = add<Core::Timer>(1000, [this] { refresh(); });
 }
 
 ThreadStackWidget::~ThreadStackWidget()
 {
 }
 
+void ThreadStackWidget::show_event(GUI::ShowEvent&)
+{
+    refresh();
+    if (!m_timer)
+        m_timer = add<Core::Timer>(1000, [this] { refresh(); });
+}
+
+void ThreadStackWidget::hide_event(GUI::HideEvent&)
+{
+    m_timer = nullptr;
+}
+
 void ThreadStackWidget::set_ids(pid_t pid, pid_t tid)
 {
     if (m_pid == pid && m_tid == tid)
         return;
     m_pid = pid;
     m_tid = tid;
-    refresh();
 }
 
 void ThreadStackWidget::refresh()

Userland/Applications/SystemMonitor/ThreadStackWidget.h

@@ -40,6 +40,9 @@ public:
 private:
     ThreadStackWidget();
 
+    virtual void show_event(GUI::ShowEvent&) override;
+    virtual void hide_event(GUI::HideEvent&) override;
+
     pid_t m_pid { -1 };
     pid_t m_tid { -1 };
     RefPtr<GUI::TextEditor> m_stack_editor;