Initial open-source release

Claude-Session: https://claude.ai/code/session_01B65qLEKnkeYTGdbwhLpjCu

src/editor/core/model/selectionStack.ts

@@ -0,0 +1,44 @@
+/**
+ * Extend / Shrink Selection (⌥↑ / ⌥↓). Extending pushes the current range on
+ * a stack and picks the innermost candidate strictly containing it; shrinking
+ * pops back. The stack is only valid while the selection is the one the last
+ * extend produced.
+ */
+export interface SimpleRange {
+  from: number;
+  to: number;
+}
+
+export interface SelectionStack {
+  /** Ranges to restore on shrink, innermost last. */
+  stack: SimpleRange[];
+  /** The selection the last extend produced. */
+  top: SimpleRange | null;
+}
+
+export const emptyStack = (): SelectionStack => ({ stack: [], top: null });
+
+const same = (a: SimpleRange | null, b: SimpleRange) => !!a && a.from === b.from && a.to === b.to;
+
+export function extendSelection(
+  st: SelectionStack,
+  current: SimpleRange,
+  candidates: readonly SimpleRange[],
+): { state: SelectionStack; next: SimpleRange | null } {
+  const base = same(st.top, current) ? st : emptyStack();
+  const next = [...candidates]
+    .filter((r) => r.from <= current.from && r.to >= current.to && (r.from < current.from || r.to > current.to))
+    .sort((a, b) => a.to - a.from - (b.to - b.from))[0];
+  if (!next) return { state: base, next: null };
+  return { state: { stack: [...base.stack, current], top: next }, next };
+}
+
+export function shrinkSelection(
+  st: SelectionStack,
+  current: SimpleRange,
+): { state: SelectionStack; next: SimpleRange | null } {
+  if (!same(st.top, current) || !st.stack.length) return { state: emptyStack(), next: null };
+  const stack = st.stack.slice(0, -1);
+  const next = st.stack[st.stack.length - 1]!;
+  return { state: { stack, top: next }, next };
+}