[huacnlee]

Problem

A popover told its trigger it was open by borrowing the selected state:

trigger.selected(trigger.is_selected() || is_open)

"Open" (a popup is showing, the trigger stays lit) and "selected" (this is the
current view, the chosen item) are two different states. Downstream
(ai-desktop/pilot) a sidebar row implements Selectable::selected to mean "this
is the current view"; used as the account popover's trigger it had to be wrapped
in a SidebarRowTrigger adapter that re-mapped selected back into open state,
because the popover would otherwise light the row as if it were the current view.

Change

Selectable gains two default methods, so nothing breaks:

fn open(self, open: bool) -> Self { self.selected(open) }
fn is_open(&self) -> bool { self.is_selected() }

Naming follows the boolean setter/reader rule in CLAUDE.md: the setter takes the
field name, the reader is is_.

The crates/base change is limited to this — Selectable lives in
crates/base/src/component_traits.rs, so the seam has to be widened there; no
base component's appearance changes.

Everything that mapped "popup is open" onto the trigger now calls open:

• gpui_base::Popover::trigger
• gpui_component::popover::Popover::trigger
• AppMenu in the app menu bar (its private is_selected helper is renamed
  is_open, which is what it always meant — the menu bar's open menu)

DropdownMenu/DropdownMenuPopover reach the trigger through
Popover::trigger, so they follow along; DropdownButton's selected is a
caller-set selection, not open state, and is left alone. Select, Combobox
and the sidebar submenu keep their own open flags and never went through
Selectable.

Button implements open explicitly and stores it in its own open field.
Open and selected look identical today, so both render through one
shows_selected_style() check — no visual change — but is_open() now reads
back what was set, and a future design can separate the two without another API
change.

Tests

• crates/base/src/component_traits.rs: a trigger implementing only selected
  still receives open state through the default open/is_open.
• crates/base/src/popover.rs: a recording trigger that keeps open and
  selected apart asserts an opening popover calls open(true) and never
  touches selected. Confirmed failing before the change
  (got [("selected", true), ...]).
• crates/component/src/button/button.rs: open and selected are stored
  apart and both paint the selected style.

cargo test -p gpui-base --lib popover              6 passed
cargo test -p gpui-base --lib component_traits     2 passed
cargo test -p gpui-component --lib                 525 passed
cargo clippy --workspace --all-targets -- -D warnings   clean
cargo fmt --check / typos                          clean

Docs

Selectable's trait doc explains what open means and when to override it.
website/component/popover.md and website/zh-CN/component/popover.md gain a
"Custom Trigger" / 「自定义触发器」 section showing a trigger that implements
Selectable and overrides open/is_open to keep the two states apart.

Breaking Changes

None. open/is_open are default methods that fall back to selected/
is_selected, so existing Selectable implementations and trigger call sites
compile and behave exactly as before.

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