Fix #3799: Alt held at the press draws a straight Pencil line at any angle

With Alt key toggles to Smooth Tool off, holding Alt when a Pencil drag
starts now draws one straight segment from the press point to the
pointer, at the angle the pointer takes, as Shift does at 45° steps.
Pressed on an end of a selected open path, the line continues the path
from that end. Both ends snap to Smart Guides as a stroke's ends do, and
the line stays open when Alt is still held at the release. Shift and Alt
together keep the 45° steps.

Before, a drag started and released with Alt held drew a closed freehand
path. Alt pressed after the drag has started still closes the path at
the release, and with Alt key toggles to Smooth Tool on, Alt still switches
to the Smooth tool. Before a Pencil press, Alt no longer shows the closing
cursor.

Docs: docs/target-app-parity.md, docs/ui-parity.md.

docs/ui-parity.md

@@ -91,7 +91,7 @@ tools, never checked side by side) isn't captured row by row.
 | Curvature tool: click/drag points, double-click or Alt-click for a corner, temporary Direct Selection, point conversion | partial | circular point markers without direction handles or duplicate markers, Anchor Size and Hide Edges regression-tested (#1236, in part); curve geometry: each stretch of three points follows the circle through them (handles along its tangent, chord/(3·cos²(α/2)) long, shorter in a turn tighter than 30°), open ends curving in, the same rule for the rubber band, dragged, inserted and converted points (#1236, in part); neighbours of a point dragged on an existing path keep their tangents, and modifiers requested in #975 remain |
 | Blend spine Anchor Point conversion and handle editing (#2544) | implemented | symmetric handles, independent drag/retraction, visible-handle policy and selection/stacking covered by tool and engine regressions; Windows native canvas checks and screenshots reviewed in PR #2643 |
 | Add/Delete/Anchor Point tools; Anchor Point tool drags segments to reshape | done | #2267: a click on a handle end retracts that handle and leaves the anchor a corner with the other one; Shift held at the press on an anchor moves it, handles and all (Shift kept down: at 45° steps) instead of pulling out handles |
-| Pencil: Fidelity, Keep selected, Alt to Smooth, edit selected paths, close by proximity | done | Alt key toggles to Smooth Tool (#2001); Shift held at the press draws a straight line at 45° steps from the Constrain Angle (part of #3799; Alt's unconstrained straight line is not done); Edit Selected Paths continues a selected path from an end and, from a stroke that starts and ends on a selected path, redraws the section between (the shorter way round a closed path, #2850); with Keep selected (Pencil and Paintbrush, on by default) off, the path a stroke draws or continues is not selected afterwards; a filled Pencil path keeps a stroke of None (#1804, part of #1679) |
+| Pencil: Fidelity, Keep selected, Alt to Smooth, edit selected paths, close by proximity | done | Alt key toggles to Smooth Tool (#2001); Shift held at the press draws a straight line at 45° steps from the Constrain Angle, and Alt held at the press (with Alt key toggles to Smooth Tool off) a straight line at any angle (#3799); Edit Selected Paths continues a selected path from an end and, from a stroke that starts and ends on a selected path, redraws the section between (the shorter way round a closed path, #2850); with Keep selected (Pencil and Paintbrush, on by default) off, the path a stroke draws or continues is not selected afterwards; a filled Pencil path keeps a stroke of None (#1804, part of #1679) |
 | Pencil: Fidelity, Keep selected, Alt to Smooth, edit selected paths, close by proximity | partial | Alt to Smooth is missing; Edit Selected Paths continues a selected path from an end and, from a stroke that starts and ends on a selected path, redraws the section between (the shorter way round a closed path, #2850); with Keep selected (Pencil and Paintbrush, on by default) off, the path a stroke draws or continues is not selected afterwards; a filled Pencil path keeps a stroke of None (#1804, part of #1679) |
 | Join tool scrubbing, Average, Join | done | Join merges the direct-selected anchors lying on one another along a path, as after Average, into one (#1533) |
 | Average and Join in one step, with a corner or smooth choice for the joined anchor | partial | `path.join` accepts `to: corner|smooth` on the merged anchor; Average and Join are still separate commands; [gap 1](gaps.md) |