Fix #5347: a floating panel's menu scrolls inside its window

The main window's menu floor was stored once for the whole egui context,
so a menu in a panel's OS window stopped at the main window's bottom bars
instead of its own edge. The floor is now kept per window, and a window
with no recorded floor uses its bottom edge. A menu list is also never
taller than the window, so the last items of a menu stay reachable in a
100 pt window, below the 120 pt minimum.

The Digits, Stylistic Sets, Save Selection and Move to Group submenus,
the libraries menu, the Color Guide's Limit to Library menu and the Layers
panel's Filter popup now scroll the same way.

docs/target-app-parity.md

@@ -677,6 +677,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - Screen readers get the value of the color and channel sliders that panels and dialogs draw (the Color panel, the Color Picker, Swatch Options, the Navigator's Zoom, the Character panel's variable font axes, the Perspective Grid and flattener options, and the sliders of the effect, tool option and transform dialogs): the number and text of the field beside each one, such as 128 for G or 50% for an opacity, with the slider's range, which extends to include a number past the slider's end. The harmony wheel's Brightness slider, which has no field, reports its brightness in percent.
   - A layer whose Layer Options › Print is off shows its name italic in the Layers panel, and while its row is dragged, as the reference application shows non-printing layers (#5739); a printing layer stays upright and ticking Print again restores the upright name (`panels::layers` test `a_layer_that_does_not_print_has_its_name_italic`).
   - Panels undock (#495): a dock tab, a panel icon or a popped-out panel's title dragged out of the dock floats that panel (in an OS window of its own on the desktop, #4135), and the strip right of the dock's tabs floats the whole tabbed group. A floating group moves by its title bar, a tab dragged out of it floats on its own, and dropped on another group's title bar or tabs it stacks with that group; dropped on a dock group's tabs, which light up under the pointer, its panels join that group, and on the group's upper or lower half they make a new group above or below it (`window.panel.dock {anchor, zone}`); dropped on the icon column they become icons there, the tabbed group's panels too (`window.panel.dock {icon: true}`); dropped anywhere else on the dock, which lights up, they go back where they live (the tabbed group or the icon column), as its × and the panel menus' Float Panel / Dock Panel do (`window.panel.float`, `window.panel.dock`). The Tools panel floats by its title bar and docks again on the window's left edge. Floating groups stack into sets: dropped on the upper or lower half of another group (a line shows where), a group joins it right above or below it, and the set's groups stack top to bottom, each showing one panel, moving together by the top group's title bar, whose × docks them all; a group dragged by its tab strip leaves the set (`window.panel.float {below, above}`). A double-click on a tab collapses a floating group to its tabs (`collapsed`), and a dock group too, a group below it moving up into the room; a second double-click, or a click on another of its tabs, expands it (`window.panel.collapse`, kept with the preferences and in saved workspaces). Sets dropped on the left edge of the icon column or of another dock column dock as dock columns (a multi-column dock, `window.panel.dock {column}`), whose groups stack, tab and tear out as a floating set's do, and whose top strip floats the column again. Floating panels, their sets and the Tools panel's position are kept with the preferences and in user workspaces, and clamped into a smaller window. Not done: panels in separate OS windows, reordering tabs by dragging, collapsing a set or dock column to icons, dock columns right of the default dock, resizing floating panels and the groups of a set.
+  - A menu in a floating panel's OS window stops at that window's bottom edge and scrolls past it, and is never taller than the window (#5347; `a_long_panel_menu_scrolls_inside_a_short_native_window`). A panel's long submenus (Digits, Stylistic Sets, Save Selection, Move to Group), the libraries menu, the Color Guide's Limit to Library menu and the Layers panel's Filter popup scroll the same way.
   - The dock's double arrow collapses Properties, Layers and Libraries to icons at the top of the icon column, popping out like the other icon panels, and expands them again (`window.collapseDock`, kept with the preferences and in saved workspaces). The icon column's own arrow (expanding the icon panels into a full column) is not done.
   - Window › Toolbars › New Toolbar… and New Toolbar… in the All Tools drawer's menu ask for a name and make a blank toolbar that floats on its own, moved by its title bar and listed under Window › Toolbars, whose item shows or hides it. The title bar's × hides it, and its double arrow shows its tools in one column or two. Its ••• button opens the All Tools drawer beside it, and tools dragged from the drawer fill it. The toolbars are kept with the workspace they were made in: switching workspaces shows that workspace's toolbars, Reset Workspace keeps them, and New Workspace starts the new workspace with a copy of them (<https://helpx.adobe.com/illustrator/using/tools.html>; `window.toolbar.new`, `window.toolbar.custom`, the `toolbar` param of `window.editToolbar` and `toolbar.addTool`/`removeTool`/`reset`; `tools_added_to_a_new_toolbar_survive_a_workspace_reload`).
   - Window › Toolbars › Manage Toolbars… and Manage Toolbars… in the All Tools drawer's menu open a dialog that lists the toolbars made with New Toolbar: a new name typed for the selected toolbar renames it, + copies it with its tools (with none selected, + makes a blank toolbar), and the trash can deletes it. Each change shows under Window › Toolbars and stays in the workspace (<https://helpx.adobe.com/illustrator/using/tools.html>; `window.toolbar.manage`, `window.toolbar.rename`, `window.toolbar.duplicate`, `window.toolbar.delete`; `manage_toolbars_renames_duplicates_and_deletes`).