Fix terminal content hidden under the scroller after a font change

[bones7456]

Problem

After changing the font size (zoom in/out), the right-most columns of the
terminal are drawn underneath the scroller and become hidden. Zooming
the font in and back out also leaves the reported column count off by a
couple of columns from what a window resize produces.

Root cause

The column/row count is computed in two places, and they disagree about
the scroller:

• The live window-resize path, processSizeChange(newSize:), uses
  getEffectiveWidth(size:), which subtracts the reserved scroller width.

• resetFont() (invoked when the font changes) computed the columns from
  the raw frame.width:

  let newCols = Int(frame.width / cellDimension.width)

So after a font change the terminal claims more columns than actually fit
in the visible (non-scroller) area, and the overflowing right-most columns
are rendered beneath the scroller.

Fix

Compute the columns from getEffectiveWidth(size:) in resetFont() too,
matching processSizeChange. The function already exists and is used on
both macOS and iOS, so this keeps the two paths consistent and guarantees
content stays clear of the scroller.

Tests

Added FontResizeColumnsTests (macOS) which:

• asserts the rendered width (cols * cellDimension.width) stays within
  getEffectiveWidth(size:) after a font change — i.e. content is not
  covered by the scroller;
• asserts the font-change path and the live-resize path report the same
  column count for the same frame.

Both tests fail on main (e.g. renderedWidth 397.5 > usableWidth 383.0,
and 53 != 51 columns) and pass with this change. The rest of the suite
is unaffected (the single pre-existing testSynchronizedOutputBlocksDisplayUntilReset
failure also reproduces on a clean main and is unrelated).

[bones7456]

Friendly ping on this one 🙂 Now rebased on latest main (sits right on top of #578, no conflicts). It's a small fix (+56/−1, 2 files) with macOS regression tests that fail on main and pass with the change. Its sibling PR #576 was already merged — this addresses the related scroller/column-drift issue after a font-size change. Happy to adjust anything if needed. Thanks for maintaining SwiftTerm!

[bones7456]

The CI failure is the pre-existing flaky testSynchronizedOutputBlocksDisplayUntilReset (SynchronizedOutputTests.swift:54), unrelated to this change. The Mac/iOS/SPM builds all pass — only that one timing-dependent test records an issue.

The exact same failure shows up on main (e.g. run 27973775699): same test, same line :54, same assertion (topLineText(...) → "NEW").hasPrefix("OLD"). A re-run should go green.

[bones7456]

Update on the CI failure: it's not flaky — testSynchronizedOutputBlocksDisplayUntilReset is a stale test that fails deterministically on main too. Commit 9446f60 removed the frozen synchronizedOutputBuffer snapshot (display blocking moved to the view layer) but never updated this test, so it still asserts the old snapshot behaviour.

Opened a separate test-only fix in #581 to keep it isolated from this font/scroller change. Once that merges, this PR's CI should go green on rebase.

[bones7456]

Rebased onto latest main (now includes the #581 test fix) and CI is green ✅. The only thing previously blocking this was that stale test, so this should be ready for review/merge whenever you have a moment. Thanks!

[migueldeicaza]

Apologies, let me review now