Add reduce motion accessibility setting

[srbsingh3]

Follows up on zed-industries/zed#48295, scoped down per feedback to ship one primitive at a time.

Summary

• Adds a reduce_motion setting ("system" | "on" | "off", default: "system")
• Queries macOS accessibilityDisplayShouldReduceMotion when set to "system"
• Applies the setting to toast slide-in animations as a first consumer
• Adds the setting to the Appearance page in Settings UI

Next steps

• Add platform support for Linux (gtk-enable-animations / portal) and Windows (SPI_GETCLIENTAREAANIMATION)
• Gate more animation call sites (with_animation is used in ~15 other places: cursor blink, edit predictions, agent panel, spinners, etc.)
• Consider lifting the check into the animate_in trait itself so all callers get it for free

Test plan

• Unit tests for serialization, round-trip, aliases, and settings store integration
• GPUI tests for should_reduce_motion with each variant
• Manual: set "reduce_motion": "on" and verify toast appears without animation
• Manual: set "reduce_motion": "system" and toggle macOS System Settings > Accessibility > Display > Reduce motion

[srbsingh3]

PR Review Summary: Add reduce motion accessibility setting

Files changed: 11 (+305/-36)
Branch: reduce-motion-accessibility-setting → main

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Critical Issues (0 found)

No critical issues were identified. The PR is well-structured and follows established codebase patterns.

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Important Issues (1 found)

1. Misleading "System" description on non-macOS platforms

[error-handling] crates/settings_ui/src/page_data.rs:1055

The settings UI description says:

"Reduce or disable animations. System uses your OS preference."

However, the should_reduce_motion() platform trait defaults to false on Linux/Windows. Users on those platforms who select "system" will see animations continue to play, with no indication that their OS preference is not being queried.

Recommendation: Update the description to mention macOS-only support for the "system" mode, or add platform-specific implementations for Linux (gtk-enable-animations / portal) and Windows (SPI_GETCLIENTAREAANIMATION). The PR's "Next steps" section already tracks this.

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Suggestions (2 found)

1. Consider making ReduceMotionSetting inner field private

[type-design] crates/settings/src/reduce_motion_setting.rs:5

The newtype pub struct ReduceMotionSetting(pub ReduceMotion) exposes the inner field, allowing callers to bypass the should_reduce_motion method and access the raw enum. Since ReduceMotion has no invalid states, this is low risk. However, making it ReduceMotionSetting(ReduceMotion) with a pub fn value(&self) -> ReduceMotion accessor would be slightly more encapsulated. This is a minor style preference.

2. Toast layer animation behavior is untested

[tests] crates/workspace/src/toast_layer.rs:261-267

The conditional animation skip (if reduce_motion { ... } else { ... }) is the only consumer of the new setting in this PR, but it has no test coverage. This is understandable given that toast_layer.rs has zero existing tests and testing element tree structure in GPUI is uncommon. The branching logic is straightforward enough that the risk is low.

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Strengths

• Follows established patterns precisely. The ReduceMotion enum matches the derive set and attribute pattern of ShowDiagnostics, WindowDecorations, and CloseWindowWhenNoItems. The ReduceMotionSetting newtype follows the RegisterSetting pattern used by BaseKeymap.

• The unwrap() in from_settings is correct. Despite CLAUDE.md saying to avoid unwrap(), the Settings::from_settings trait explicitly documents that it should panic on missing defaults. Every other Settings implementation does the same. The default value "reduce_motion": "system" is present in default.json.

• Excellent public API. The free function should_reduce_motion(cx: &gpui::App) -> bool provides a clean, ergonomic interface. Consumers don't need to know about ReduceMotionSetting, ReduceMotion, or the get_global machinery.

• Thorough test suite (7.5/10). Covers default values, deserialization of all valid inputs, rejection of invalid inputs, serialization round-trip, all three setting variants, settings store integration with JSON parsing, and registration ordering edge cases.

• Clean unsafe FFI. The macOS accessibilityDisplayShouldReduceMotion implementation follows the exact same pattern as the adjacent should_auto_hide_scrollbars, and Objective-C nil messaging semantics provide implicit safety.

• Smart serde aliases. #[serde(alias = "true")] on On and #[serde(alias = "false")] on Off anticipate a common configuration mistake. Tests correctly verify that bare boolean true/false (without quotes) are rejected.

• Proper VS Code import handling. Setting reduce_motion: None correctly falls back to the default since VS Code has no equivalent setting.

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Recommended Action

1. Consider updating the settings UI description to indicate that "system" detection currently only works on macOS
2. Merge — no blocking issues found. This is a well-implemented accessibility feature that integrates cleanly across all layers (platform → GPUI → settings → UI)

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Review performed by Claude Code using 4 specialized agents: code-reviewer, pr-test-analyzer, silent-failure-hunter, and type-design-analyzer.