LibVT: Reset scrollbar after switching to/from the alternate buffer

We did not call the history change callback after switching to the
alternate screen buffer, which caused the scrollbar to not change its
maximum value. If we already had lines in the scrollback buffer, this
meant that we could drag the scrollbar, which then tried to access
non-existent lines from the scrollback.

Fixes #8581

Userland/Libraries/LibVT/Terminal.cpp

@@ -126,9 +126,11 @@ void Terminal::alter_private_mode(bool should_set, Parameters params)
                 dbgln_if(TERMINAL_DEBUG, "Switching to Alternate Screen Buffer");
                 m_use_alternate_screen_buffer = true;
                 clear();
+                m_client.terminal_history_changed(-m_history.size());
             } else {
                 dbgln_if(TERMINAL_DEBUG, "Switching to Normal Screen Buffer");
                 m_use_alternate_screen_buffer = false;
+                m_client.terminal_history_changed(m_history.size());
             }
             m_need_full_flush = true;
 #else
@@ -148,11 +150,13 @@ void Terminal::alter_private_mode(bool should_set, Parameters params)
                 m_normal_saved_state = m_current_state;
                 m_use_alternate_screen_buffer = true;
                 clear();
+                m_client.terminal_history_changed(-m_history.size());
             } else {
                 dbgln_if(TERMINAL_DEBUG, "Switching to Normal Screen Buffer and restoring state");
                 m_current_state = m_normal_saved_state;
                 m_use_alternate_screen_buffer = false;
                 set_cursor(cursor_row(), cursor_column());
+                m_client.terminal_history_changed(m_history.size());
             }
             m_need_full_flush = true;
 #else