change: deck split shows exactly the pinned panels (no leftover column)

Pinning 2 of 3 panels previously showed a 3rd "leftover" tab-column, which
read as all-three-pinned and was confusing. Now the split view shows exactly
the pinned panels (pin 2 -> 2 columns, pin 3 -> 3). Adds an always-available
View > Side-by-side panels submenu of checkable toggles as the way to pin a
panel while already in split view (the right-click-tab gesture only works in
tabbed mode). Tests assert exactly-N-columns and the menu-pin path; the win
fixture now resets deck state so tests don't depend on persisted layout.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_ui_structure.py

@@ -20,6 +20,11 @@ def win(app):
         w = MainWindow()
     except Exception as e:  # GL/mpv/display unavailable, etc.
         pytest.skip(f"MainWindow could not be constructed here: {e}")
+    # Deterministic deck state regardless of any persisted side-by-side layout
+    # (construction restores deck_pinned from QSettings).
+    for _p in w._deck_panels:
+        _p._pinned = False
+    w._refresh_deck_layout()
     yield w
     w.close()
     w.deleteLater()
@@ -63,10 +68,42 @@ def test_deck_stack_exists(win):
     assert win._deck_stack.currentWidget() is win._control_deck
 
 
-def test_pinning_two_panels_switches_to_split(win):
+def _split_columns(win):
+    """Widgets of the splitter actually mounted in the layout (not findChild,
+    which can return a stale deleteLater'd splitter)."""
+    from PyQt6.QtWidgets import QSplitter
+    item = win._deck_split_layout.itemAt(0)
+    spl = item.widget() if item else None
+    assert isinstance(spl, QSplitter)
+    return [spl.widget(i) for i in range(spl.count())]
+
+
+def test_pinning_two_panels_shows_exactly_two_columns(win):
     # Pin two panels directly (avoid the toggle handler so no QSettings write
     # leaks into other test windows) and refresh.
+    from PyQt6.QtWidgets import QTabWidget
     win._tab_export._pinned = True
     win._tab_crop._pinned = True
     win._refresh_deck_layout()
     assert win._deck_stack.currentWidget() is win._deck_split_container
+    cols = _split_columns(win)
+    assert len(cols) == 2                                    # only the pinned ones
+    assert not any(isinstance(c, QTabWidget) for c in cols)  # no leftover tab-column
+
+
+def test_side_by_side_menu_pins_third_panel(win):
+    # In split mode the View ▸ Side-by-side menu is the way to pin a 3rd panel
+    # (there's no tab bar to right-click). Suppress the QSettings save via the
+    # _deck_loading guard so this doesn't leak into other windows.
+    win._tab_export._pinned = True
+    win._tab_scan._pinned = True
+    win._refresh_deck_layout()
+    assert len(_split_columns(win)) == 2
+    act = next(a for a, p in win._deck_pin_actions if p is win._tab_crop)
+    win._deck_loading = True            # suppress _save_deck_layout
+    try:
+        act.trigger()                   # simulate clicking the menu item
+    finally:
+        win._deck_loading = False
+    assert win._tab_crop._pinned is True
+    assert len(_split_columns(win)) == 3