renderer: a selection drag that leaves the renderable cannot scroll it, so a selection cannot span more than one screen #1930

Summary

While a selection drag is active, the renderer clamps the drag endpoint to the owning renderable's visible grid and delivers drag events to whatever renderable is under the pointer. It never scrolls the renderable that owns currentSelection. So a drag that leaves the renderable's bounds — the normal way to select more than one screen — cannot extend the selection beyond the visible area.

Confirmed with a terminal pane in opencode; the same applies to any scrollable, selectable renderable such as ScrollBox.

Evidence

Raw mouse capture of the real gesture (WezTerm, opencode 2.2.5): press at the bottom row, drag upward.

 43  Cb=0   x=230 y=52 M  press
 44  Cb=32  x=230 y=51 M  drag ┐
 …                            │ y descends 52 → 1
 66  Cb=32  x=212 y=1  M  drag ┘  ← clamped at row 1, outside the pane
 90  Cb=0   x=215 y=9  m  release

No scroll occurs at any point. Control case: wheel with the button held and the pointer inside the pane scrolls normally, so local scroll in the embedded terminal and the renderer's scroll dispatch both work during a drag. The only missing behaviour is the out-of-bounds case.

Where it is

• packages/core/src/renderables/EmbeddedTerminal.ts, onSelectionChanged clamps every endpoint:

  const point = (x: number, y: number) => {
    if (y < 0) return { x: 0, y: 0 }
    if (y >= this.height) return { x: this.width - 1, y: this.height - 1 }
    return { x: Math.max(0, Math.min(this.width - 1, x)), y }
  }

• packages/core/src/renderer.ts, processSingleMouseEvent: a drag calls updateSelection(maybeRenderable, x, y) and then sendMouseEvent(maybeRenderable, …) — the event goes to the hit target, not to the selection owner, so the owner is never told that the pointer left it.

• _latestPointer is private, so a host cannot read the pointer position and implement autoscroll itself.

Proposal

Either of these makes the gesture work; the first also fixes scrollboxes:

1. Renderer autoscroll. While currentSelection.isDragging and the pointer is outside the owner's bounds, scroll the owner on a timer (roughly every 50–100 ms, proportional to distance past the edge) through a minimal contract on the renderable — for example scrollBy(delta). EmbeddedTerminalRenderable already has the operation internally (contextEmbeddedTerminalScroll), and scrollable containers have equivalents. Keep the selection endpoint pinned to the edge row while the content moves under it.
2. Expose the gesture. Dispatch drag events to the selection owner as well as the hit target, with unclamped coordinates (and/or expose the pointer publicly), so a host can implement autoscroll itself.

Notes

• Not opencode-specific: any host with a scrollable selectable region has this.
• Related: input: wheel events are decoded as drag while a mouse button is held, so nothing scrolls during a drag #1911 — retracted; the wheel is decoded correctly, the problem is that during an edge drag the pointer is outside the pane, so the wheel is hit-tested elsewhere. The terminal-pane symptom is tracked in tui: terminal pane viewport is frozen during a selection drag, so a selection can never extend past one screen opencode#61053.
• No new protocol is required; the scroll primitive already exists.