The Timeline's "Edit Gradient…" on a gradient property (Gradient Fill /
Gradient Stroke ▸ Colors, Gradient Overlay) only painted its label. It now
opens a Gradient Editor (prop.editGradient): opacity stops above a preview
bar and colour stops below. Click to add a stop (it takes that track's
own value there), drag or type a Location to move it, set its opacity or
colour (egui's colour picker), Delete removes it (never the last of a
track, never while typing). The tracks stay in order while editing and the
selection follows its stop.
The editor works on a draft. OK sets the property once with prop.set at
the composition time it opened, snapped to the frame, with
timeBase: "comp" (offset and stretched layers get the key where it was
opened): one undo step, and nothing at all when the draft is unchanged, so
OK adds no keyframe. Cancel and Escape change nothing; Escape with the
picker open closes only the picker. On an animated gradient only the
keyframe at that time changes; when its stop counts no longer match a
neighbouring keyframe's (gradients only blend with equal counts) the
editor says so. The draft is saved in the typed form, so one colour stop
and each stop's RGBA survive.
prop.editGradient checks its target is a gradient property before the UI
opens anything, so agents get an error for a wrong one. New Gradient
color_at / opacity_at sample one track each (sample() multiplies opacity
in, which would apply it twice to a new stop).
Tests: ui_gradient_value (fails on main: the value had no automation id),
ui_gradient_editor (11), gradient_editor and keyframe unit tests.
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