ADR 0277: Draggable Chat Content Width

• Status: Accepted
• Date: 2026-09-16
• Deciders: PI-Desktop desktop UI maintainers
• Amends: D058, D101, the collapsed-sidebar 640px band
• Related: 04-ux/01-ui-ia · 04-ux/07-ui-design-system · 04-ux/08-component-spec · 08-meta/decisions-log (D439) · E2E-CHAT-content-width-handles · E2E-208

Context

The conversation column, empty-home stack, and composer were capped at 760–768px (640px while the sidebar was collapsed) so a wide shell would not become a low-density reading surface. Code, tables, diagrams, and tool output then wrapped inside a second 720px assistant cap, and there was no way to spend the extra pane width when the user wanted it.

A Comfort/Wide toggle was rejected: two presets cannot match monitor size, work-panel squeeze, or a one-off wider reading of a diff.

Decision

1. The transcript, empty-home stack, and composer share one preferred max width, default 760px, persisted as AppSettings.chatContentMaxWidth. Absent or invalid values keep 760. The live band is always min(available pane minus 24px gutters, preferred), so a squeezed sidebar or work panel compresses the band without rewriting the preference.

2. Two 12px handles sit on the left and right edges of that centered band, from the top of .chat-surface down to the composer dock. Both handles change the same width (1px pointer → 2px band) so the column stays centered. Drag minimum is 560px, or the available pane if smaller.

3. Rest: the handles are invisible. Hover/focus: a short 2×40px capsule mixed from --ds-text-primary (18% on dark, 12% on light). Drag: both capsules lengthen to 56px at a slightly stronger mix. Double-click resets to 760. Arrow keys step 16px (Shift 32px); Home restores 760; End expands to the pane; Escape cancels an in-flight drag.

4. User bubbles stay min(82%, 600px). Assistant, tool, permission, ask, review, and turn-outcome rows follow --chat-prose-max-width, which tracks the band. The collapsed-sidebar 640px ceiling is removed.

5. Renderer only: no protocol, storage schema, host, or IPC change. Host-core already preserves unknown settings keys.

Consequences

• Users who never touch the handles keep today's 760px column, including after collapsing the sidebar (the pane grows; the band does not jump to 640px).
• A saved 1100px preference on a large window compresses cleanly when the work panel opens, then returns when it closes.
• Handle hit targets can sit near the minimap when the band is almost full pane width; the 24px gutter keeps both usable.

Rejected alternatives

• Comfort / Wide presets: cannot match the live pane or a one-off width.
• Independent left/right insets: un-centers the column and fights the composer, which is also centered.
• Full-bleed to the pane edge by default: reopens the over-wide reading surface the 760px cap was there to prevent.