Selection API (window.getSelection, Range) for text selection

[wangnaihe]

Context

Per docs/vscode-compat.md Phase A (P0). Text selection is fundamental to any text editor. Monaco Editor and every text-based UI depends on `window.getSelection()` and the `Range` API.

Requirements

Selection API

• `window.getSelection()` → `Selection` object
• `Selection.anchorNode` / `anchorOffset` — where selection starts
• `Selection.focusNode` / `focusOffset` — where selection ends
• `Selection.isCollapsed` — true if no text is selected (cursor only)
• `Selection.toString()` — selected text content
• `Selection.collapse(node, offset)` — set cursor position
• `Selection.selectAllChildren(node)`
• `Selection.removeAllRanges()` / `addRange(range)`

Range API

• `document.createRange()` → `Range`
• `Range.setStart(node, offset)` / `setEnd(node, offset)`
• `Range.startContainer` / `endContainer` / `startOffset` / `endOffset`
• `Range.collapsed`
• `Range.getBoundingClientRect()` → position of selected text (for cursor/highlight rendering)
• `Range.cloneContents()` / `extractContents()` / `deleteContents()`

Rendering

• Render selection highlight (blue background) over selected text
• Render text cursor (blinking caret) at collapsed selection position
• Handle mouse drag to create selection
• Handle Shift+Arrow keys to extend selection
• Handle Ctrl/Cmd+A to select all
• Handle Ctrl/Cmd+C to copy selected text (integrates with Clipboard API)

Implementation Notes

This is tightly coupled with text layout — need to map pixel coordinates (mouse click) to text node + character offset. Requires glyph-level hit testing from the font/layout engine.

References

• MDN Selection API
• MDN Range API
• docs/vscode-compat.md §2.1 — Selection API

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