iOS: distinct sticky-lock cue for glass toolbar modifiers (double-tap lock vs one-shot arm) - #5604

On iOS 26 the Liquid Glass accessory bar (#5536 / #5579) made the one-shot-armed and double-tap sticky-locked modifier states render identically: both used .prominentGlass() with a .systemBlue tint, so you could not tell a locked modifier from an armed one. The pre-iOS-26 flat style had always distinguished the locked state with a 2pt white stroke; that cue was lost on the glass path. Reported in dogfood: double-tapping a modifier to sticky-lock it showed no visible difference.

This restores the locked cue as a white capsule border drawn on the button's layer, over the glass. AccessoryActionButton gains an isStickyLocked flag that syncs a layer-level border (cornerRadius = bounds.height / 2, .continuous curve, white, 2pt) from didSet and layoutSubviews. Drawing at the layer level (not via UIButton.Configuration.background.strokeColor) composites the border on top of Liquid Glass regardless of how the glass material renders its own background, so the cue reads clearly over glass. It mirrors the pre-26 white stroke, and adds zero intrinsic width so it does not fight the bar's min-width sizing. The border is set unconditionally from sticky, so a button going locked then armed then resting never keeps a stale border. It is applied only on iOS 26; the pre-26 flat path keeps its existing background-config stroke (no doubled border).

Three states now read distinctly:

• resting: plain .glass()
• one-shot-armed (single tap): .prominentGlass() + blue, no border
• sticky-locked (double tap): .prominentGlass() + blue + white capsule border

Dogfood: single-tap Ctrl shows the armed look (blue glass, no border); double-tap Ctrl shows the clearly-locked look (blue glass with a white capsule border around it). The simulator may not composite the glass material, so the locked-over-glass read is best confirmed on device.

Branching and merge sequencing: this is based off feat-ios-toolbar-reorder-builtins (#5579), which carries the glass styling being fixed. Merge #5579 first, then this. The in-flight button-min-width PR #5599 (feat-ios-accessory-min-width) edits the exact same two functions on the same line, so it overlaps and will conflict on merge: both touch accessoryButtonConfiguration and applyAccessoryButtonStyle in TerminalInputTextView.swift. The border cue here was deliberately chosen to add zero width so it composes with #5599's narrower buttons; sequence #5599 and this PR together and resolve the trivial overlap in applyAccessoryButtonStyle.

Verified: clean iOS simulator Debug build (cmux-ios scheme, isolated derivedDataPath). Not a device build, no xcodebuild test. Autoreview clean.

🤖 Generated with Claude Code

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Summary by cubic

Fixes the iOS 26 Liquid Glass toolbar so sticky‑locked modifiers show a clear white capsule border, making them distinct from one‑shot armed. Pre‑iOS‑26 styling stays the same.

• Bug Fixes

  • Added isStickyLocked to AccessoryActionButton to sync a 2pt white layer border (cornerRadius = height/2, continuous curve) and clear it when unlocked.
  • Border updates in didSet and layoutSubviews; applied only on iOS 26. Earlier iOS keeps the existing background stroke.
  • Visual states: resting (glass), armed (prominent glass + blue), locked (prominent glass + blue + white border).

• Migration

  • Merge feat-ios-toolbar-reorder-builtins first. Expect a small overlap with feat-ios-accessory-min-width in applyAccessoryButtonStyle and accessoryButtonConfiguration.

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