Image Trace: pressing and holding the eye button shows the source image

The Image Trace panel had no way to compare a trace with its source
image except changing View and changing it back.

The panel now has an eye button to the right of View, enabled with an
Image Trace object selected. While it is held down, the canvas draws the
selected Image Trace objects with their source image, whatever their
View, and draws their View again on release. The document, its View and
the undo history are unchanged. The renderer takes the objects in
RenderOptions::trace_source, which is also part of the canvas frame key
and the raster-effect cache key.

docs/target-app-parity.md

@@ -788,6 +788,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 - **Image Trace, thin closed outlines:** generated `fit::snap_area_tests` verify nonzero interiors after straightening, opposite closed-boundary winding and Noise controls; see [quality notes](image-trace-quality.md#thin-closed-outlines-with-snap-curves-to-lines). Scores and hours unchanged; open-piece fitting is unchanged.
 - **Image Trace, flattened slivers:** in a Color, Grayscale or Black and White trace with Fills, a traced region whose outline straightens onto one line (a single pixel, or a one-pixel row along an edge, its pieces shared with the regions beside it) is left out instead of becoming a selectable path with no area, which paints nothing. Snap Curves To Lines straightens more outlines, so it leaves out more of these. Without Fills, the region's 1 px stroke draws it, and it is kept. Code-drawn tests in `crates/trace`.
 - **Image Trace, Auto Color rare colours:** the Automatic palette keeps colours too rare for its area count when they are distinct (Oklab) and drawn as coherent regions, at most six, protected from mixed-colour merging and Noise; code-drawn stem, highlight, low-contrast and scattered-noise tests (see [quality notes](image-trace-quality.md#auto-color-keeps-rare-colours)).
+- **Image Trace, source image on hold:** while the eye button next to the Image Trace panel's View is held down, the canvas shows the selected Image Trace objects' source image whatever their View, and shows their View again on release, with no change to the document or the undo history.
 - **Image Trace, large images:** images over 1500 px on a side trace with a warning in the result and the status bar; the anchor limit is 8,000,000 (2,000,000 on the web), measured at about 90 bytes an anchor (see [quality notes](image-trace-quality.md#large-images)).
 - **Image Trace, Preview:** the Image Trace panel's Preview checkbox, before Trace, is on by default, and a setting change traces the selected Image Trace object again at once. Off, changes wait in the panel until Trace applies them as one undo step, or until Preview is turned on again. Preview is kept with the UI preferences, and Reset to Default leaves it.
 - **Image Trace small contours:** closed fits too small to form an editable subpath retain their positioned outline, including holes. Source-generated island, hole/winding and stepped-region regressions cover Corners and Noise; defaults, parity scores and estimates are unchanged.

docs/ui-parity.md

@@ -274,6 +274,7 @@ empty), without opening the dialog.
 | Panels outside the app window (another monitor) | missing | single OS window |
 | Edit Toolbar (•••): the All Tools drawer opens beside the toolbar by its button (Escape or a click outside closes it), dims the tools already in the toolbar; while it is open, tools drag from it onto the toolbar (between two tools, or onto one to join its group), from the toolbar into it, and within the toolbar; each toolbar keeps its edits | done | #1018; `window.editToolbar`, `toolbar.addTool`/`removeTool`/`reset`; the drawer's Reset (Illustrator has it in a drawer menu); dragging a toolbar button moves the tool it shows, not its whole group; New/Manage Toolbars missing; tool flyouts are as wide as their longest translated name and shortcut need (#1616) |
 | Quick Image Trace presets trace at once and leave the panel as it was; the Image Trace button (Control bar, Properties) opens the editable controls | done | Per the public documentation, choosing a preset traces the image with it straight away, and the panel is opened with the Image Trace button in the Control panel or in Properties' Image Trace section (<https://helpx.adobe.com/illustrator/using/image-trace-results-optimization.html>). The controls show Threshold, Paths, Corners and Noise with Advanced open; `panels/image_trace.rs` headless tests (`a_quick_trace_leaves_the_panel_as_it_was`, `sketch_quick_trace_reveals_all_adjustments_and_keeps_the_source` for the button's route). No algorithm parity claim. |
+| Image Trace panel: holding the eye button next to View down shows the selected Image Trace objects' source image on the canvas | done | Per the public documentation, the control next to View is pressed and held to compare the trace result with the source image (<https://web.archive.org/web/20250831090623/https://helpx.adobe.com/illustrator/using/image-trace.html>). On release the objects show their View again; the document, its View and the undo history are unchanged. `RenderOptions::trace_source`; `panels/image_trace.rs` headless test `holding_the_eye_button_shows_the_source_image_until_released`, render test `trace_source_draws_the_source_image_of_the_listed_objects`. The Control bar and Properties have no such button |
 | Live trace result bounds and selection highlights exclude the retained bitmap; source-image views retain it | done | All TraceView modes and rotated result bounds regression-covered; ordinary hidden group behavior retained |
 | Image Trace panel: the Preview checkbox before Trace chooses whether a setting change traces the selected Image Trace object again at once | done | Per the public documentation, Preview shows the result of the current settings and Trace traces with them (<https://web.archive.org/web/20250831090623/https://helpx.adobe.com/illustrator/using/image-trace.html>). Preview is on by default and kept with the UI preferences; Reset to Default leaves it. Off, changes wait in the panel until Trace applies them as one undo step, or until Preview is turned on again. `ui.set {imageTracePreview}`; `panels/image_trace.rs` headless tests (`with_preview_off_a_slider_change_waits_for_trace`, `turning_preview_on_traces_the_changes_that_waited`, `preview_turned_on_with_nothing_selected_traces_when_the_object_is_selected_again`, `settings_that_wait_stay_with_their_object`, `preview_is_kept_with_the_preferences_and_reset_to_default_leaves_it`) |
 | Toolbar without group labels by default, one or two columns; All Tools drawer by flyout group with shortcuts, its menu (Reset, Basic, Advanced) and Show section | done | #1751; a preference brings the labels back; `toolbar.show` sets the Fill/Stroke, Coloring, Draw Mode and Screen Mode controls |