[PawelTroka]

Why

Addresses the browser 2D interaction gap in #292: ordinary real and complex charts currently replace one static PNG. This is a separate workstream from draft #294.

Draft — do not merge without the owner's explicit approval.

What changes

• Opt-in interactive2D on the existing POST /api/v2/chart returns bounded real-curve or complex-value samples through the existing evaluator, request-local custom definitions and isolated worker. Default API callers and implicit expressions retain PNG rendering.
• Coordinate-aware Canvas 2D plotting provides pointer-anchored wheel zoom, drag pan, touch pinch, keyboard and toolbar controls, reset and explicit bounds. New viewports request mathematical samples; resize and complex display changes reuse cached numbers.
• Add function supports up to eight real overlays, each with independent visibility/removal and a frozen expression/custom-function snapshot. Sequential sampling batches publish atomically; cancellation, errors and late responses preserve the last committed plot.
• Existing layers can become empty when navigating outside their mathematical domain without blocking other functions. A narrowly scoped allowEmpty2D opt-in permits this; new Draw/Add operations still reject an entirely undefined initial domain.
• Complex None/Linear/Logarithmic contours, validated spacing, magnitude color direction, axes and nearest-sample inspection. PNG export includes the current viewport, visible functions, axes and legend.
• Bounded midpoint checks break common off-grid poles without disconnecting steep straight lines. Undefined values become holes. Tick and hover precision follows the visible span.

Verification

Current source head: 75aef98191149838d13a02c3e808c9f5802271d8.

Current-head check	Result
Windows API regression suite	152 passed, 1 existing documented 3D skip, all 23 new chart cases passed; published API smoke passed — job, diagnostics
Real Windows API + Chromium/Firefox acceptance	16/16 passed, no skips — run, screenshots and PNG exports
Real custom functions and worker cancellation	26/26 passed, no skips/failures, Chromium and Firefox — run, browser evidence
Published frontend build, audit and browser suite	262/262 passed, no skips; build/audit passed, 0 vulnerabilities — run, diagnostics
Combined 2D/WebGL Windows acceptance	20/20 passed, no failures/errors/skips, actual WebGL in Chromium and Firefox — run, source identifiers and screenshots/exports

The final combined artifact retains 32 unique PNG stages, including all ten switching stages in each browser. Those twenty switching images were visually inspected: 2D axes/labels/legends are complete, WebGL visibly rotates/zooms, and exports contain the plotted content. Source identifiers and the downloaded artifact digest were verified.

Coverage includes actual wheel/drag/keyboard, native Chromium two-finger pinch, per-layer source snapshots, cached resize/color controls, domain-limited layers, cancellation/stale-result guards, malformed responses, request bounds, and decoded PNGs with nonblank plot pixels. API tests cover independent numerical values, grid orientation, singularities, custom definitions and isolation, sampling/precision limits, legacy PNG compatibility and cancellation.

PR #294 compatibility

Target: 9fc1b7949fab5dab4cd3ce04cfef2bf944511b01, based on master fe1aba49b5f9538355ffaaa3d1683f69a23d2d9c.

• Numeric 2D dispatch applies only to explicit real/complex functions; the surface contract remains separate.
• A reproducible disposable-checkout workflow applies the immutable feat(web): interactive f(x,y) 3D surface charts with numerical sampling and WebGL #294 diff and a reviewed integration bridge. Chart.tsx and the solution documentation inventory need explicit reconciliation; conflict-free merging is not claimed.
• The bridge preserves lazy SurfacePlot, quality selection, cached resize, camera controls, WebGL reporting and export. This standalone PR does not add Plotly or duplicate SurfacePlot.
• Full switching exposed an asynchronous resize callback after the surface was removed. The bridge includes a narrow lifecycle guard and handles obsolete resize rejections while retaining errors for a live plot. The strict no-pageerrors assertion remains intact.
• The additional acceptance scenario switches real → WebGL → real → complex on one page, checks independently expected numbers and renderer disposal, exercises camera/viewport changes, and exports nonblank PNGs. Actual WebGL is required in both browsers.
• Clean LF and actual CRLF preparations produce the same combined tree: 281fdbabeb40001adfc362cb02405d321d00c5b9. Bridge SHA-256: 881efd2f00e1768cc1b69aea334ab85c6c29a0cfcc8b97d897f77f16e517c542. CI retains these identifiers in webgl-294.json alongside screenshots and results.
• Validation changes neither source PR nor master and merges neither draft PR.

Remaining differences

Implicit charts retain PNG/manual bounds. Real overlays are bounded at eight; complex charts retain one field, as on desktop. Scripted point series, desktop autoscale variants, per-axis wheel modifiers, arbitrary style/property dialogs and print dialogs remain separate.

Desktop exponential contours are omitted because their current magnitude-dependent loop has problematic/nonterminating edge cases. The browser's lighter color direction follows documented endpoint semantics instead of reproducing the desktop formula discrepancy; exact color-pixel parity is not claimed. Sampling is finite and bounded, so narrow features may require zooming. API hosting remains Windows-dependent. Visual review also records the inherited #294 camera/aspect framing: a tall paraboloid can extend beyond the frame and crop ground-axis labels; 3D framing improvements remain in that workstream.

Architecture, contracts, commands and parity boundaries: WEB-2D-INTERACTIONS.md.

All requested chart, API and browser gates above passed on the current head, and the complete four-job Windows regression matrix passed. At the latest check, 21 of 22 repository workflows were successful; the separate release identity/candidate evidence workflow was still running, with no reported failure.

This PR remains draft. No existing or new PR has been merged.