#770: Proportional Scrubbing of Timeline values (Ctrl+Alt / Cmd+Opt)

Problem: After Effects 26.2 spreads a Ctrl+Alt (Cmd+Opt) hot-text scrub
over the selected layers by selection order (first unchanged, last the
whole change, the ones between even shares). Our multi-layer scrub
(#491) always moved every selected layer by the same change.

Fix: prop.set takes `proportional` (with `selected` + `offset`); each
participating layer, the scrubbed one included, gets index/(n-1) of the
change in selection order, in one undo step; the first layer is left
alone (no key added). The Timeline value editor sends it while Ctrl+Alt
(Cmd+Opt) is held; that chord no longer fine-scrubs or unlinks Scale.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/ui-parity.md

@@ -17,7 +17,7 @@ feel and platform differences, and the 26.x interface changes are not in yet.
 |---|---|---:|---|
 | Menu bar | After Effects' menus, order, submenus and labels; native on macOS; keyboard-operable in-window bar | 95% | 587 entries in `crates/engine/src/menus.rs`; Adobe-service entries omitted by design; `ui_menus` tests (#279) |
 | Panels and workspaces | Every After Effects panel; all standard workspaces; drag-to-dock, tabs, floating panels, maximise | 75% | `ui_tabs`, `ui_panels`; dropping a panel on a stacked panel doesn't make a tab (#302); stacked-panel headers (#301); saved workspaces don't persist (#320); no panels on a second monitor (#486) |
-| Timeline | Switches, modes, parent & link, twirl-downs, reveal shortcuts (U, UU, P, S, R, T, A, E, M, F, Alt+Shift…), pick whips, graph editor, work area, markers | 75% | `ui_keyframes`, `ui_timeline_*`; Graph Editor jitter and hidden buttons (#456, #457); rotation can't go negative by dragging (#569); bulk keyframing (#355); expand precomps in place (#601); Proportional Scrubbing (26.2) missing |
+| Timeline | Switches, modes, parent & link, twirl-downs, reveal shortcuts (U, UU, P, S, R, T, A, E, M, F, Alt+Shift…), pick whips, graph editor, work area, markers | 75% | `ui_keyframes`, `ui_timeline_*`; Graph Editor jitter and hidden buttons (#456, #457); rotation can't go negative by dragging (#569); bulk keyframing (#355); expand precomps in place (#601); Proportional Scrubbing (26.2: Ctrl+Alt / Cmd+Opt scrub of hot text spreads the change over the selected layers by selection order, #770) matched for layers selected as a whole, not yet for individually selected properties |
 | Composition viewer | Selection, handles with direction-aware cursors, rulers, guides, grids, snapping, channels, exposure, region of interest, snapshots, 3D views, Spacebar hand | 70% | `ui_viewer`, `ui_viewers`; Cmd/Ctrl +/− doesn't zoom (#570); inaccurate snapping (#352); anchor point grab on Linux (#492); no 3D transform gizmos (#392, #347); percentage guides (26.5); copy frame to clipboard (26.3) |
 | Tools panel | All After Effects tools; long-press flyouts; Tool Creates Shape / Mask; Fill / Stroke options | 80% | #213, #227 fixed; pen and vertex tools on motion paths (#553) |
 | Nudging and modifiers | Arrow nudge (1 px, Shift 10 px), Alt/Option duplicates and slips, Shift constrains | 75% | #290 nudging fixed; Linux multi-layer edits (#491) |