Forms: an option list stays inside the window; media ratios are checked in one place

The floating list under a Select, Combobox or MultiSelect was at least its
trigger's width with no ceiling, so a long choice ran it past the window
and its rows never ellipsized. It now stops at the window's width less the
window margin on each side, so gpui snaps it inside. The camera preview
and the model viewer check their ratio through primitives::checked_ratio.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -65,7 +65,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - The macOS open panel runs out of process too. Escape does not reach it; the harness sends it `cancel:`.
 - gpui's `test-support` swaps its executor: `block` fails on a real dispatcher. It stays behind this crate's `test-support` feature, never on for the gallery.
 - A tracked `FocusHandle` is a Tab stop only when built with `.tab_stop(true)`. `tab_index` on the div reaches only handles gpui makes itself. `primitives::tab_stop` keeps one per id.
-- Floating lists go below their anchor, or above when only above has room (`forms::options::float`). gpui's own switch keeps the anchor point, so a flipped list would cover its trigger.
+- Floating lists go below their anchor, or above when only above has room (`forms::options::float`). gpui's own switch keeps the anchor point, so a flipped list would cover its trigger. An option list is at least its trigger's width and at most the window's less `window_margin` each side, so gpui can snap it inside and its rows end in an ellipsis; wider, gpui pins it left and the rest runs off.
 - A trigger that opens a list goes through `forms::select::listing`: toggle, keys, blur and popup live there, for Select, TabOverflowMenu and Breadcrumb alike.
 - A palette goes through `navigation::palette::Palette`: query field, grouped rows, cursor, keys and focus live there. CommandPalette, QuickOpen, QuickSwitcher, SearchPalette and QuickLauncher only build rows.
 - A menu goes through `menus::menu`: rows, submenus, keys, focus and presses outside live there, for DropdownMenu, OverflowMenu, SplitButton, ContextMenu, SearchableMenu and MenuBar. A press on the host's own box is the host's to handle, so a trigger toggles and a right click moves a context menu without a close and reopen in one event.