feat(plugin-ui): chat-style input submit, scroll follow/observe/jump, markdown node

[gegnep]

Summary

Three additive, default-off plugin UI capabilities, plus two bugfixes found
while exercising them from a real plugin:

• input submitOnEnter prop: Enter submits, Shift+Enter inserts a newline,
  Ctrl+Enter still submits (existing behavior unchanged when off)
• scroll stickToBottom / onScroll(offset, maxOffset) / scrollToBottomRev
  props for follow-scroll, scroll observation, and an explicit jump-to-bottom
• ui.markdown node type registering the existing md4c MarkdownView, with the
  source cached per slot so streaming re-renders only re-parse on change
• fix: runStream slots (cap 4/host) were only reclaimed on host teardown, so
  four short-lived streams permanently exhausted the cap; exited processes now
  mark their cancel token and startStream sweeps them
• fix: MarkdownView measured labels unwrapped, under-allocating its height and
  overlapping sibling rows when used alongside other children in a flex

Motivation

Plugin panels that stream content (chat-style UIs) currently can't offer
Enter-to-send composers, can't follow or observe scroll position, and can only
render plain labels. All three are small exposures of capability the codebase
already has. Developed for and exercised by a real plugin (a Claude chat
panel: https://github.com/gegnep/noctalia-plugins); happy to adjust naming or
semantics to fit the project's direction.

Type of Change

• New feature
• Bug fix
• Breaking change
• Refactoring
• Build / packaging

Related Issue

—

Testing

• Rebased onto current main; just format clean on every commit (applied
  per-commit via rebase --exec).
• Full nix build including all test targets; ui_tree_reconciler_test links
  the markdown control (its source list gained markdown_view/builders/md4c —
  caught by a real build, not hypothetically).
• Live-tested extensively via the chat panel plugin on Niri: all three
  Enter-key behaviors; follow-scroll during streaming; scroll-up disengages
  the stick and onScroll reports offset/max; scrollToBottomRev snaps back;
  markdown (headings/lists/inline code) streaming without re-parse churn;

  4 sequential runStream invocations keep working; markdown views mixed with
  sibling rows lay out without overlap.

• Note: scrollToBottomRev applies once on the first reconcile that sees it
  (a fresh scroll starts at the bottom when the prop is present).

Manual Coverage

• Tested on Niri
• Tested on hyprland
• Tested on sway
• Tested on other WM
• Tested on Multiple Monitors

Screenshots / Videos

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed.
• I ran the relevant build or test commands.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge (plugin-prop docs live
  on the external docs site; nothing in-repo to update).
• I added or updated assets/translations/en.json, or this PR adds no
  new user-facing strings. (No new strings.)
• I did not edit non-English translation files.
• I used the existing canonical names for config keys, IPC names, paths,
  and identifiers.

Additional Notes

Plugin-API feedback from building against it, offered for the
docs or as follow-ups rather than included here:

• Manifest select options with an empty value are silently dropped
  (plugin_manifest.cpp), and noctalia plugins lint doesn't flag the
  now-unselectable declared default — a lint warning candidate.

Portions of these patches were implemented with AI assistance (Claude Fable 5 orchestrating GPT -5.5)
under human review and testing; the repo has no stated policy on this, so disclosing rather than assuming.

[gegnep]

rebased onto main

[gegnep]

rebased onto main

[gegnep]

rebased over the drag-and-drop merge

[gegnep]

dropped the slot-reaping commit, superseded by 3065b05

[gegnep]

rebased over df40cac; dropped the meson hunk, superseded by the test refactor

[ItsLemmy]

1. [P1] Version the new plugin API before exposing it — src/scripting/ui_prelude.h:29-30

(https://github.com/gegnep/noctalia/blob/a166bcc20b574e194f038613e68085fd615c5d75/src/scripting/ui_prelude.h#L29-L30)
ui.markdownand the new input/scroll props expand the public plugin contract, but the PR does not increment kCurrentPluginApiVersion. Plugins therefore cannot declare these capabilities as their minimum requirement: an older shell will accept the plugin, then fail on ui.markdown or ignore the unknown props. Current main is API 19, so this should introduce API 20 after rebasing. The required separate-repo update is also missing:
official-plugins/noctalia.d.luau (https://github.com/noctalia-dev/official-plugins/blob/main/noctalia.d.luau#L354-L378) does not declare ui.markdown.

2. [P1] Defer scrollToBottomRev until the new scroll extent exists — src/ui/ui_tree_reconciler.cpp:1649-1653

(https://github.com/gegnep/noctalia/blob/a166bcc20b574e194f038613e68085fd615c5d75/src/ui/ui_tree_reconciler.cpp#L1649-L1653) applyProps()runs before child reconciliation and layout. This reads the previous maxScrollOffset(), and setScrollOffset() immediately clamps to that stale range. Consequently, a fresh scroll starts at offset 0 despite the PR description, and adding content while incrementing the revision jumps only to the previous bottom unless stickToBottom is also enabled. Store a pending jump and consume it in ScrollView::doLayout() after recomputing
m_maxScrollOffset.

3. [P2] Include scale in the markdown parse cache key — src/ui/ui_tree_reconciler.cpp:1606-1612

(https://github.com/gegnep/noctalia/blob/a166bcc20b574e194f038613e68085fd615c5d75/src/ui/ui_tree_reconciler.cpp#L1606-L1612) MarkdownView::setMarkdown()bakes m_scale into font sizes, spacing, padding, and radii, but reconciliation calls it only when the text changes. When a surface’s fractional scale changes or a plugin surface moves between differently scaled outputs, ordinary props rescale while the retained MarkdownView
remains at the old scale. Cache (text, scale) or invalidate markdown slots when UiTreeReconciler::setScale() changes.

[gegnep]

All three points addressed; rebased onto main (624363e30, clean).

1. Plugin API 20 : new trailing commit adds
   kPluginUiPropsPluginApiVersion = 20 and retargets
   kCurrentPluginApiVersion. Declaration-only, matching the 18/19
   precedent; the manifest range check enforces it. The separate-repo
   updates are up as drafts, ready once this merges:
   noctalia.d.luau typings in Declare ui.markdown and plugin API 20 props official-plugins#34
   and the docs (version ledger + declarative-ui) in Document plugin API 20 ui additions noctalia-docs#213.
2. scrollToBottomRev : reworked as suggested: the reconciler now
   calls a new ScrollView::requestScrollToBottom(), which stores a
   pending flag (and cancels an in-flight fling so a stale tween can't
   drag the view back). doLayout() consumes the flag right after it
   recomputes m_maxScrollOffset, folded into the existing
   stick-to-bottom pin, with an unconditional clear so a request issued
   at the bottom can't fire on a later pass. A fresh scroll with the
   prop present now opens at the bottom.
3. Markdown scale : the parse cache keys on (text, scale). The
   scale check sits outside the text-presence gate, so a re-render that
   omits text (retained-prop convention) still picks up a rescale.

Tested live against a patched shell: fresh chat opens at the bottom,
mid-stream rev bumps land on the new bottom, a jump during a fling
holds, and markdown rescales when the panel moves between differently
scaled outputs.