popover: Add offset and optional anchor-aligned arrows

[huacnlee]

Description

Popover now supports anchor + offset + arrow: callers can adjust the trigger gap and enable an arrow aligned to any of the eight existing anchors, including TopCenter and BottomCenter. The arrow follows the anchor's edge and leading/center/trailing alignment, with an inset to avoid rounded corners. arrow defaults to false; spacing and arrows preserve the selected anchor and its window-clamping behavior.

Popover::new("details")
    .anchor(Anchor::TopCenter)
    .offset(px(8.))
    .arrow(true)
    .trigger(Button::new("details-trigger").label("Details"))
    .child("Popover content")

Correct the documentation: Anchor::TopLeft names the popup's own anchor and opens below the trigger. Positioning measures the actual trigger rectangle and uses current-frame bounds when the trigger moves. The compact Anchor story demonstrates all eight anchors with small trigger buttons and a shared Arrow checkbox, unchecked by default. English and Chinese docs are updated.

Public API

gpui-component

• popover::Popover::offset(self, offset: impl Into<Pixels>) -> Self — set the trigger-to-surface gap, or trigger-to-arrow-tip gap; defaults to 0.25rem.
• popover::Popover::arrow(self, arrow: bool) -> Self — show the themed arrow aligned to the existing anchor; defaults to false.

gpui-base

• Popover::offset(self, offset: Pixels) -> Self and Popup::offset(self, offset: Pixels) -> Self — set the outward gap for the selected anchor; defaults to zero.
• Popover::on_position(self, callback: impl Fn(ResolvedPosition, Bounds<Pixels>) + 'static) -> Self and Popup::on_position(self, callback: impl Fn(ResolvedPosition, Bounds<Pixels>) + 'static) -> Self — observe resolved popup and trigger geometry before content prepaint for custom presentation.
• Positioner::on_position(self, callback: impl Fn(ResolvedPosition) + 'static) -> Self — observe resolved geometry before children prepaint.

No new Popover placement/alignment API, enum re-exports, JavaScript methods, or TypeScript declarations. Base supplies geometry; Component draws the arrow.

Breaking Changes

The existing horizontal-center anchors now attach to the corresponding trigger edge instead of using the old top-left fallback:

- .anchor(Anchor::LeftCenter)  // Legacy top-left fallback
+ .anchor(Anchor::LeftCenter)  // Right of the trigger, vertically centered
- .anchor(Anchor::RightCenter) // Legacy top-left fallback
+ .anchor(Anchor::RightCenter) // Left of the trigger, vertically centered

The six top/bottom anchors retain their default positioning. Anchors clamp into the window without flipping.

How to Test

• cargo test -p gpui-base -p gpui-component --lib --locked — 1,548 tests passed.
• cargo clippy -p gpui-base -p gpui-component --lib --locked
• cargo build -p gpui-component-story -p gpui-base-examples --bins --locked
• cargo run -p gpui-component-story -- Popover: in the final Anchor section, toggle the Arrow checkbox and click each anchor button. Check leading, center, and trailing arrow alignment and spacing, with and without arrows.
• cargo run -p gpui-base-examples -- popover: exercise the unstyled popover lifecycle.

Regression coverage includes eight anchor positions, explicit gaps, moving triggers, no implicit flipping, arrow anchor alignment, joins, and default positioning. Native testing was on Linux; macOS and Windows were not tested locally.

Checklist

• Read and followed CONTRIBUTING.md.
• Reviewed the changes, including AI-generated code.
• Built and ran the affected Popover story on Linux.