feat(editor): Ctrl+Cut inserts a transparent band

[lukejmorrison]

Independent of clip-out (#111) and Snap defaults (#113). Branched from tobi/omasnap main.

With Cut armed (X), hold Ctrl (or press Ctrl+X) to insert space instead of collapsing it.

Behavior

• Same drag as Cut: pick an axis, preview a band, release to commit.
• Insert grows the image and fills the gap with transparent pixels (converts to ARGB if needed).
• Toolbar icon swaps to a split-plus while Ctrl is down; live preview is a green band with a plus instead of the hatched X.
• Annotations past the seam shift out. Undo is the same Cut op with insert set.
• Full-extent remove is still a no-op (would leave nothing). Insert may grow past the current extent. Empty always bails.

Model

CutOp gains insert (JSON insert: true only when set, so old logs stay valid). composeCuts / applyCutOp branch to insertBand vs removeBand. Replay uses shiftForInsert for annotations and grows the selection.

Ctrl at mouse-press arms insert; the 3px drag lock records it (QTest mouseMove has no modifiers, so press-time is the source of truth, OR'd with live Ctrl if the real compositor sends it).

Tests

• runCutSmoke — insertBand, applyCutOp, composedLogicalSize growth, shiftForInsert
• runCutMappingSmoke — Ctrl+X arms Cut, Ctrl+drag logs an insert, composed source grows with a transparent gap

make check is green (build, offscreen smoke, clang-tidy).

[Copilot]

🟡 Changes recommended

The insert-band implementation and UI hint logic have confirmed edge-case inconsistencies (insertBand end-clamping and modifier handling) that can produce incorrect behavior or misleading UI.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds an “insert band” variant to the Cut tool in the editor: when Cut is armed, holding Ctrl turns the gesture into inserting a transparent band (expanding the image) instead of removing and collapsing it, with corresponding UI hints, serialization support, and smoke tests.

Changes:

• Extend CutOp with insert, serialize/deserialize it in the operation log, and branch cut replay to insert vs remove.
• Implement transparent-band insertion in the cut engine and update annotation/selection shifting on replay.
• Update editor UI (status text, toolbar icon, drag preview) and add smoke coverage for Ctrl+Cut insertion behavior.

File summaries

File	Description
tests/cut-smoke.cpp	Adds unit-style smoke assertions for insert shifting, insertBand, applyCutOp insert, and composedLogicalSize growth.
tests/cut-mapping-smoke.cpp	Adds end-to-end editor smoke for Ctrl+X arming cut and Ctrl+drag producing an insert + transparent gap.
src/icons.cpp	Adds a dedicated toolbar icon for cut-insert (split-plus).
src/editor.hpp	Adds a test accessor for composed source and declares cutInsertHint().
src/editor.cpp	Implements Ctrl-to-insert behavior across status/tooltips/icon swap, drag logging, replay shifting, and preview rendering.
src/cut.hpp	Extends CutOp with insert and declares insert/shift helpers.
src/cut.cpp	Implements insertBand/applyCutOp/shiftForInsert and updates composeCuts/composedLogicalSize for inserts.
src/capture.cpp	Persists cut.insert in JSON only when true; reads it back on load.
README.md	Documents Ctrl (or Ctrl+X) to insert a transparent band when using Cut.
docs/editing-model.md	Updates the editing model docs to describe cut insertion semantics.

Review details

Suppressed comments (2)

src/cut.cpp:75

• insertBand() vertical path has the same issue as the horizontal one: end is never clamped, so the inserted band size can exceed the actual remaining extent when end is out of bounds (e.g., from ceil/ratio rounding). Clamp end and derive the band size from clamped coordinates before allocating the output image.

  const int width = image.width();
  start = std::clamp(start, 0, width);
  QImage out(width + band, image.height(), image.format());
  out.setDevicePixelRatio(image.devicePixelRatio());
  out.fill(Qt::transparent);

src/editor.cpp:3969

• keyReleaseEvent() also treats Meta as the insert modifier (resetting status on Meta release), but insert detection uses only Qt::ControlModifier. Keeping Meta here can leave the Cut UI hint/status out of sync with actual behavior.

  if (tool_ == Tool::Cut &&
      (event->key() == Qt::Key_Control || event->key() == Qt::Key_Meta) &&
      !cutDragActive_) {
    setStatus(QStringLiteral("Cut: drag across a band to remove it"));

• Files reviewed: 10/10 changed files
• Comments generated: 3
• Review effort level: Lite

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[tobi]

Reviewed and tested locally. Added endpoint clamping, consistent Ctrl-only modifier handling, and out-of-range regression coverage.

[Copilot]

🟡 Changes recommended

Insert currently can’t grow past the bottom/right edge because both the drag coordinates and insertBand() clamp to the current extent, which conflicts with the stated “insert may grow past the current extent” behavior.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 2
• Review effort level: Lite