render: interactive tab switching with per-surface active-tab state

[joestump]

What

Tabs are now interactive. The tab bar joins the focus ring; when it holds focus, left/right (or vi-style h/l) switch the active tab and its content renders in place of the old always-first-tab behavior.

How

• Per-surface state: Surface.activeTabs maps Tabs component ID → active index, lazily initialized. A missing entry means the first tab.
• Focus ring: collectFocusables collects the tab bar itself (when it has tabs) and recurses only into the ACTIVE tab's subtree, so focus can never land on a component hidden inside an inactive tab. switchTab re-collects the ring after each switch and keeps focus on the bar.
• Chrome: the active title renders bold (Heading) normally and reverse-video (ButtonFocused) while the bar holds focus — same monochrome discipline as button focus chrome, so host themes win.
• Preservation: applyComponents leaves activeTabs untouched (like focus, it's user state the server must not clobber); deleteSurface wipes it. Out-of-range indices left behind by a merge that shrank the tab list clamp back to the first tab at read time (activeTab), so rendering never indexes past the end.
• Text fields keep h/l: the new key cases fall through to text-field editing when the focused component isn't a tab bar.
• New exported accessor Surface.ActiveTab(id) mirrors Focusables() for hosts/tests.

Tests

render/tabs_test.go (all green with go build ./... && go vet ./... && go test -count=1 ./...):

• tab bar joins the focus ring; empty Tabs stays out of it
• left/right and h/l switching, wrap-around at both ends, no switching while blurred
• active-title chrome: reverse-video only while the bar holds focus, bold otherwise
• active tab survives Apply merges (updated active-tab content renders, no reset to first)
• out-of-range index after a tab-list-shrinking update clamps to the first tab
• focusables inside inactive tabs leave the ring and swap in on switch
• h/l still type into a focused text field; arrows remain no-ops there
• deleteSurface + re-create resets to the first tab

Docs: updated README.md and docs/wire-format.md "Not yet" lists. The website/ copy of the docs is deliberately untouched — the website docs "Not yet" update is deferred to the post-merge docs pass.

Fixes #45

🤖 Posted on behalf of @joestump by Claude.

[joestump]

Adversarial review of the tab-switching implementation against #45. Verified at head e61d02f with go build ./... && go vet ./... && go test -count=1 ./... all green locally.

Blockers

None. Specifically hunted and cleared:

• Key-routing regressions (render/render.go:246-260): the new "left","h" / "right","l" cases reproduce the old default-case behavior byte-for-byte when the focused component is not a tab bar — h/l still type into a focused TextField via key.Text, arrows stay no-ops there (key.Text is empty), and buttons are unaffected. Standalone only intercepts ctrl+c/esc/q, so nothing upstream swallows the new keys.
• Wrap arithmetic (render/render.go:408): ((activeTab+delta)%n + n) % n is correct for delta ±1 from a clamped base in [0,n).
• Panic safety in switchTab (render/render.go:395-417): the focused tab bar is always reachable from the root via a path that does not traverse its own tabs, so the post-switch collectFocusables always re-collects it and the focusIdx restore loop always finds it — no out-of-range focusIdx.
• State preservation (render/composite.go:71-94): applyComponents leaves activeTabs untouched (matching the focus-preservation precedent), deleteSurface wipes it, and the read-time clamp in both renderTabs and collectFocusables prevents indexing past a shrunken tab list. Focus preservation composes correctly with clamping (a focused button inside a removed tab falls back to focusIdx = 0).
• Focus-ring hygiene (render/render.go:569-578): only the ACTIVE tab's subtree is walked, so focusables inside hidden tabs leave the ring; empty Tabs stay out of it.
• Chrome discipline: Heading (bold) / ButtonFocused (bold+reverse) only — monochrome, host themes win.
• Test quality: render/tabs_test.go asserts observable behavior (rendered content via ansi.Strip, ring membership, exact chrome bytes), not mere execution, and covers every Done-when item: switching (arrows + h/l), wrap-around, blur guard, Apply survival, out-of-range clamp, delete/re-create reset, and the h/l-into-TextField non-regression.

Ticket compliance: all #45 Done-when items are met. The website/ "Not yet" copy is correctly deferred to the post-merge docs pass per repo policy, and the PR body says so.

Nits (non-blocking)

1. Stale active-tab "resurrection" — render/render.go:423-429 (activeTab). The clamp is read-time only and never rewrites s.activeTabs, so this sequence: user selects tab index 2 → a merge shrinks the list to 2 tabs (renders tab 0) → a later merge grows it back to 3+ tabs → the view silently jumps back to index 2 with no user input. Arguably a feature (selection survives a transient shrink), but it contradicts the "falls back to the first tab" phrasing in the doc comments. Either clear/rewrite the stored entry when the clamp fires, or document the resurrection as intended.
2. Stale test name — render/placeholders_test.go:16 still names the test TestTabsRenderTitleBarAndFirstChild while its updated comment describes "the default Tabs rendering … the ACTIVE tab". TestTabsRenderTitleBarAndActiveChild (or similar) would match.

Approving-quality change: no verified correctness defects, ticket contract satisfied.

🤖 Posted on behalf of @joestump by Claude.

[joestump]

Merged origin/main into this branch (370c53f) so it merges cleanly again. Main had landed five PRs touching the same files since the branch forked — #48 (createSurface no-op), #49 (ChildList templates), #51 (Modal), #52 (editable inputs), #53 (InputSubmitted/ChoiceSelected dispatch) — and the resolution is a union of features: nothing from either side was dropped.

Conflicted files and how each was resolved

• render/render.go — the big one:
  • Surface keeps all state fields: this branch's activeTabs alongside main's fieldValues/checkValues/choiceValues/sliderValues/choiceCursor/openModals/scope.
  • isInteractive now covers Buttons, the five inputs, Modals, and non-empty Tabs bars; collectFocusables walks only the active tab's subtree, so inputs and modals inside inactive tabs are excluded from the focus ring (and only an open modal's content joins it).
  • Key routing precedence: left/right step a focused Slider (main) and switch tabs on a focused tab bar (branch), keyed off the focused component type; h/l switch tabs only when the tab bar holds focus and remain literal runes on a focused TextField/DateTimeInput (main's rune-edit path wins — the auto-merge had left duplicate case "left"/case "right" arms that would not compile, now unified); enter/space/esc keep main's button/checkbox/choice/modal/InputSubmitted behavior unchanged.
• render/composite.go — deleteSurface clears every state map including activeTabs; applyComponents preserves active-tab state across merges alongside focus/modal/edited-value preservation, with out-of-range indices clamping to the first tab at read time.
• README.md / docs/wire-format.md — merged the feature descriptions from both sides; the "not yet" lists are now empty since Editable CheckBox / ChoicePicker / Slider / DateTimeInput (finish #15) #42–createSurface: honor or explicitly document ignoring (theming/catalog hints dropped silently) #47 have all landed.

Tests added (render/tabs_test.go) pinning the new cross-feature interactions:

• TestTabsLeftRightDisambiguateSliderAndTabBar — left/right step a focused slider without switching tabs, and switch tabs on the focused tab bar without stepping the slider.
• TestTabsInactiveTabExcludesInputsAndModals — a TextField and a Modal in inactive tabs are excluded from the focus ring, swapping as the active tab changes.

Gate: go build ./..., go vet ./..., gofmt -l . (clean), go test -race ./... all green locally; CI on 370c53f passed (https://github.com/joestump-agent/a2tea/actions/runs/29659231142). #53's event dispatch paths are untouched and their tests still pass.

🤖 Posted on behalf of @joestump by Claude.