Fix Environment/composer overlap and clamp responsive workspace panel widths

[redxzeta]

Parent: #125
Priority: P0

Problem

The current docked workspace layout allows the Environment panel and right dock to consume/overlay core chat/composer space instead of participating in a safe responsive layout.

This is not only a visual-density complaint. Hands-on Playwright testing reproduced actual overlapping hit regions.

At 1440×900

The docked Environment panel occupied roughly:

x = 1128 → 1440
width ≈ 312px
z-index = 20
position = absolute

Composer controls simultaneously occupied the same region, including approximately:

voice: x = 1143 → 1171
send:  x = 1179 → 1207

The Environment panel can therefore cover/intercept composer controls.

At 1024×768

With left sidebar + right dock open, the layout allowed approximately:

left sidebar   256px
main workspace 176px
right dock     591px

With the Environment panel open, its controls also occupied the same horizontal region as composer model/effort controls.

A developer workspace should not permit persisted panel widths to collapse the primary working surface to ~176px or cover model/send controls.

Goal

Make workspace panel geometry responsive and deterministic so the primary chat/composer surface always retains a usable minimum width and docked panels never overlap interactive composer controls.

Required work

• Audit the layout ownership of:
  • left thread sidebar
  • main chat/thread surface
  • Environment panel
  • right dock
  • composer footer
• Replace accidental absolute-overlay behavior where a panel is conceptually docked.
• Define minimum usable main-workspace width.
• Clamp persisted right-dock width against current viewport size.
• Decide explicit breakpoint behavior when all surfaces cannot coexist:
  • automatically collapse a secondary panel,
  • convert Environment to an overlay/drawer that does not cover composer actions,
  • or otherwise preserve the main interaction surface.
• Preserve user-resizable panels at widths where the layout has sufficient space.
• Ensure resize persistence cannot restore an impossible geometry after window shrink.

Acceptance criteria

• At 1440×900, Environment + right dock + sidebar can be opened without any Environment hit target overlapping composer controls.
• At 1024×768, opening the right dock does not reduce the main workspace to an unusable sliver; the chosen responsive behavior activates instead.
• Model selector, effort selector, voice control, send control, and composer extras remain pointer- and keyboard-accessible whenever visible.
• Window resize from a wide persisted layout to 1024px resolves into a valid layout automatically.
• Browser tests assert bounding-box non-overlap for representative composer controls and panel surfaces.
• Browser tests cover both 1440×900 and 1024×768 or equivalent representative breakpoints.
• Existing Terminal/right-dock state continuity is preserved unless a deliberate responsive collapse occurs.

Non-goals

• Do not redesign Environment contents.
• Do not remove the right dock.
• Do not solve this by simply hiding core composer controls.
• Do not make all panels fixed-width if responsive clamping is sufficient.

[redxzeta]

Additional evidence from the 2026-08-29 read-only Playwright audit on current built-from-scratch (85a9c882c):

• At 1024×768, with the 256px project sidebar and Environment open, the composer/work surface was roughly 436px wide.
• At 800×700, the composer/work surface collapsed to roughly 212px while the 256px project sidebar and ~286px Environment panel remained present.
• EnvironmentPanel.tsx explicitly reserves ENVIRONMENT_DOCKED_CONTENT_INSET_PX = 312.
• The header itself adapts via HEADER_COMPACT_BREAKPOINT = 700, so the remaining gap is shell/panel adaptation rather than header controls.

Suggested acceptance refinement: protect a minimum usable chat/composer width and automatically collapse/overlay secondary chrome before the primary work surface falls below that threshold. This current reproduction did not show page-level horizontal scrolling; the failure mode is primary-workspace starvation.