fix(viewer): preserve gap toggles in organize and comparison views

M3 / PR #3101: avoid stale organizer anchors and synchronize the active comparison gap choice across both panes through menu and control paths. Three regression tests fail before these fixes and pass after them. Document the gap helpers and test contracts.

parity/acrobat-features.toml

@@ -852,9 +852,9 @@ area = "B"
 tier = "P0"
 milestone = "M3"
 status = "shipped"
-commands = ["view.continuous"]
+commands = ["view.continuous", "view.page_gaps"]
 tools = []
-evidence = ["crates/ui-egui/tests/view.rs::page_gaps_stay_closed_at_fractional_device_scales", "crates/ui-egui/tests/view.rs::page_gaps_toggle_joins_edges_without_changing_the_document", "crates/ui-egui/tests/view.rs::page_gaps_default_survives_restart_and_applies_to_new_documents", "crates/ui-egui/tests/view.rs::page_gaps_menu_joins_pages_without_a_shadow_at_the_seam", "crates/ui-egui/tests/view.rs::page_gaps_work_for_mixed_sizes_and_two_page_rows", "crates/ui-egui/tests/view.rs::continuous_layout_stacks_pages_vertically", "crates/ui-egui/tests/view.rs::page_display_commands_switch_layouts", "crates/ui-egui/tests/view.rs::default_page_display_is_used_for_new_documents", "crates/ui-egui/src/canvas.rs::fit_page_and_height_hold_still_while_mixed_page_sizes_scroll_past", "crates/ui-egui/tests/view.rs::touchpad_swipe_glides_on_after_the_fingers_lift_and_stops", "crates/ui-egui/tests/view.rs::touching_the_touchpad_again_or_turning_animations_off_stops_the_glide", "crates/ui-egui/tests/view.rs::rapid_wheel_notches_accelerate_and_follow_touchpad_input", "crates/ui-egui/tests/view.rs::changing_continuous_layout_resets_wheel_acceleration"]
+evidence = ["crates/ui-egui/tests/view.rs::page_gaps_in_organizer_do_not_move_the_document_scroll_position", "crates/ui-egui/tests/view.rs::page_gaps_comparison_menu_preserves_the_active_panes_choice", "crates/ui-egui/tests/view.rs::page_gaps_comparison_control_preserves_both_states", "crates/ui-egui/tests/view.rs::page_gaps_stay_closed_at_fractional_device_scales", "crates/ui-egui/tests/view.rs::page_gaps_toggle_joins_edges_without_changing_the_document", "crates/ui-egui/tests/view.rs::page_gaps_default_survives_restart_and_applies_to_new_documents", "crates/ui-egui/tests/view.rs::page_gaps_menu_joins_pages_without_a_shadow_at_the_seam", "crates/ui-egui/tests/view.rs::page_gaps_work_for_mixed_sizes_and_two_page_rows", "crates/ui-egui/tests/view.rs::continuous_layout_stacks_pages_vertically", "crates/ui-egui/tests/view.rs::page_display_commands_switch_layouts", "crates/ui-egui/tests/view.rs::default_page_display_is_used_for_new_documents", "crates/ui-egui/src/canvas.rs::fit_page_and_height_hold_still_while_mixed_page_sizes_scroll_past", "crates/ui-egui/tests/view.rs::touchpad_swipe_glides_on_after_the_fingers_lift_and_stops", "crates/ui-egui/tests/view.rs::touching_the_touchpad_again_or_turning_animations_off_stops_the_glide", "crates/ui-egui/tests/view.rs::rapid_wheel_notches_accelerate_and_follow_touchpad_input", "crates/ui-egui/tests/view.rs::changing_continuous_layout_resets_wheel_acceleration"]
 notes = "Page gaps, shadows and outlines can be hidden through the page-display menu, with a persisted default in Preferences and matching UI control options; page margins in the PDF are unchanged. Continuous scrolling remains the default for PDFs that don't ask for a layout (Preferences ▸ Default page display); pages stack with no snapping. Fit page and Fit height fit the largest page, so the zoom holds still across mixed page sizes. Rapid line-wheel notches gain a bounded, constant-space arrival-rate boost (#1072); isolated notches still use the chosen Scroll speed, and page units stay unboosted. A touchpad swipe glides on after the fingers lift on Wayland (#531; macOS glides by itself; Windows and X11 report no lift through winit), unless Animation effects is off; a click, key, zoom or new scroll catches it. Not yet: fingers resting on the touchpad don't catch it (needs Wayland hold gestures, which the vendored winit doesn't bind)"
 
 [[feature]]