Pen: Auto Add/Delete on a selected path

[theironchef]

Fixes #311.

Problem: the Pen had no Auto Add/Delete. Over a segment or an anchor of a selected path, it showed the plain Pen cursor and a click started a new path on top of it. The "Disable Auto Add/Delete" preference (Prefs::disable_auto_add_delete) was stored but never read. In Illustrator, the Pen acts as the Add Anchor Point tool over a selected path and as the Delete Anchor Point tool over one of its anchors. Shift while hovering, or that preference, turns this off.

Change

• crates/tools/src/pen.rs, new auto_add_delete(cx, p, mods, tol), for the selected, editable, non-guide paths:
  • an anchor within the Pen's 5 px tolerance → path.removeAnchor;
  • otherwise a segment within it → path.insertAnchor at that point (both are the commands the Add and Delete Anchor Point tools already run);
  • None when Shift is held or the preference turns it off.
• The Pen's button-down uses it between paths:
  • after the blend-spine clicks and after "click an end of a selected open path to continue it", so the ends still continue the path;
  • before starting a new path.
• cursor shows PenAdd / PenDelete for the same cases.
• While a path is being drawn, clicks go on drawing it, as before.
• ToolContext::auto_add_delete (tools) carries the preference. The engine fills it with !prefs.disable_auto_add_delete in with_tool_cx.
• The module doc says how it works.

Tests

• New pen::tests::clicking_a_selected_path_adds_or_deletes_an_anchor:
  • add on a segment and delete on an anchor, with their cursors;
  • an end still shows the continue cursor;
  • Shift and the preference turned off give the plain cursor and a new path;
  • a click while drawing goes on drawing.
• It fails without the change (the cursor over a segment is Pen, not PenAdd).
• New engine test pen_click_on_a_selected_path_adds_an_anchor_unless_disabled, through the session:
  • a click on a selected 3-anchor path's segment gives 4 anchors;
  • after prefs.set {"key": "disableAutoAddDelete", "value": true}, the same click starts a second path and leaves 3.
• cargo test -p vectorcraft-tools: 189 passed. cargo test -p vectorcraft-engine: 1251 unit tests and the integration tests passed.
• cargo clippy -p vectorcraft-tools -p vectorcraft-engine --all-targets -- -D warnings (Rust 1.99): clean.
• cargo fmt --all -- --check: clean.
• Checked by hand in a release build on Windows with real mouse input, on a selected 3-anchor open path with the Pen:
  • hovering a segment showed the add cursor, and a click added an anchor there (3 → 4, History "Add Anchor Point");
  • hovering an anchor showed the delete cursor, and a click deleted it (4 → 3, "Delete Anchor Point");
  • an end still showed the continue cursor;
  • a Shift-click on a segment started a new path, as did a plain click with disableAutoAddDelete on. With it off again, the click added an anchor.

I didn't run the full cargo xtask ci (wasm step) here. The Pen's hint bar text is unchanged.

Two things are left out. Paths inside a selected group aren't covered: only directly selected paths count, as for continuing a path. Clicking the last anchor of the path being drawn, to retract its handle, isn't handled either.

This touches different lines of pen.rs than #310 (Alt mid-drag) and #241 (Alt-click on an anchor).

🤖 Generated with Claude Code

[bflatastic]

Thanks @theironchef, merged! The Pen now adds and deletes anchors on a selected path, Shift turns it off while held, and Preferences › General › Disable Auto Add/Delete is finally read.

I rebased it on main and added one commit on top. The Pen now reuses the Add/Delete Anchor Point tools' hit tests and action builders (draw2::anchor_in / segment_in, insert_anchor / remove_anchor) instead of carrying a second copy, which also drops a per-hover allocation. The ROADMAP is updated too.

Live check: a click on a selected path's segment went from 3 to 4 anchors, and a click on an anchor went back to 3. History shows "Add Anchor Point" / "Delete Anchor Point" with the + / − cursors. A Shift-click, or prefs.set {disableAutoAddDelete: true} over MCP, started a new path instead.