Fix stale rows when repainting in place while scrolled back

[evertjr]

Problem

The terminal's update range is recorded in buffer.y space, relative to yBase. updateDisplay turns it into an invalidation rect as if buffer row y were screen row y, but drawTerminalContents maps screen rects back to buffer rows through yDisp:

let firstRow = displayBuffer.yDisp+Int ((boundsMaxY-dirtyRect.maxY)/cellHeight)

Those agree only while the viewport is pinned to the bottom. Scrolled back by k rows, a change at buffer row y renders at screen row y + k while the invalidation still covers screen row y, so the rows that actually changed are never repainted. They stay stale until a resize or another scroll forces a full redraw.

Scrolling output masks it: scroll() ends with updateRange(scrollTop) and updateRange(scrollBottom), so on a full-height screen rows 0 and rows-1 are both dirty and the rowEnd == rows - 1 branch invalidates everything anyway. What stays exposed is in-place repainting, where the update range remains a strict subset. Agent TUIs that redraw a status block several times a second (Claude Code, Codex) hit this constantly: scroll up a few lines while one is working and the visible rows stop tracking the buffer.

The alternate screen is unaffected, it has no scrollback so yDisp == yBase always.

Fix

Invalidate the whole view when yDisp != yBase.

The pinned-to-bottom case keeps the existing partial rect untouched, including the restricted-region extension from #582. It is less blunt than it reads: drawTerminalContents still repaints only the rows intersecting the dirty rect, each read from its correct yDisp-relative line.

CoreGraphics path only. Metal builds its own metalDirtyRange a few lines below and is not affected.

Tests

Added a regression test: 20-row view, scrolled back 3 rows, cursor parked on row 2, then a write that neither scrolls nor moves the cursor. Update range is (2,2), and before the fix the invalidated rect is [256, 288] while the change renders at [224, 240]. Fails on main, passes with the fix. Rest of the suite is green.

[migueldeicaza]

This looks good, just a couple of small edits coming in in a second

[migueldeicaza]

Thank you for your contribution!