fix: stable formatting for conditional expression as if/else body

[bartlomieju]

Summary

Fixes a "Formatting not stable" non-idempotency: a single-statement if/else if body that is a conditional (ternary) expression could format differently on each run.

Reproduction

With the deno config preset:

function w(n) {
  if ("play" === n) {
    var w = 1;
    play({ ids: [b] });
  } else if ("queue" === n) playing() ? queue({ ids: [b] }) : queue({ ids: [b] });
}

Formatting once produces a wrapped ternary inside added braces; formatting again collapses it to one line:

   } else if ("queue" === n) {
-    playing()
-      ? queue({ ids: [b] })
-      : queue({ ids: [b] });
+    playing() ? queue({ ids: [b] }) : queue({ ids: [b] });
   }

So the output is not a fixed point. In a consumer that re-formats to verify stability (e.g. deno fmt, which retries up to 5 times), this surfaces as Formatting not stable. Bailed after 5 tries.

Root cause

The inline form } else if ("queue" === n) playing() ? ... : ...; is 82 columns, over the 80 limit, so:

1. the ternary is measured at the inline (post-header) column ~26 and wraps;
2. the wrapped body is multiple lines, so WhenNotSingleLine adds braces;
3. once braced, the body moves to its own line at indent 4 (58 columns), where it would fit on one line — but it has already been wrapped against the pre-brace column.

The brace decision is re-evaluated via the printer's look-ahead/reevaluation, but update_state_to_save_point in dprint-core restores writer position without clearing already-resolved conditions, so the conditional expression's break decision (resolved at the inline column) sticks. Binary, call, and member-expression bodies don't hit this because they lay out inside a single new_line_group that re-fits cleanly on reprint; the conditional expression emits its SpaceOrNewLine breaks at the top level, outside such a group.

Fix

Wrap the single non-block statement body in a new_line_group so its line-break decisions are re-measured at the body's final indentation, matching the behavior of the constructs that were already stable. The change is one line in gen_conditional_brace_body.

Tests

• New spec tests/specs/deno/Deno_ConditionalExpressionStatementBody.txt. The spec runner formats twice (format_twice: true), so this asserts idempotency; it fails on main and passes with the fix.
• Full suite passes (649 tests).

Scope / caveats

This was found while investigating denoland/deno#26840 (large minified bundle bailing with "Formatting not stable"). This patch fixes the ternary-body class of instability. That particular file has additional, independent non-idempotency (e.g. single-statement bodies under multi-line-wrapped if headers whose comma-sequence body sits at the width boundary), driven by the maintain-source-line-breaks behavior when preferSingleLine is false. Those are not addressed here; this PR is one self-contained increment.