fix(install): read native child logs with an explicit UTF-8 codec

scripts/install.ps1

@@ -600,7 +600,10 @@ function Write-NpmDebugLogTail {
     }
     $tail = $null
     try {
-        $tail = Get-Content -LiteralPath $logPath -Tail $TailLines -ErrorAction Stop
+        # npm writes this log itself, as UTF-8 with no BOM. Get-Content has no
+        # BOM to sniff and falls back to the ANSI code page, so the codec has to
+        # be stated. See the note above _Invoke-NativeWithTimeout.
+        $tail = Get-Content -LiteralPath $logPath -Tail $TailLines -Encoding UTF8 -ErrorAction Stop
     } catch {
         Write-Warn "Could not read npm debug log ${logPath}: $($_.Exception.Message)"
         return
@@ -3299,13 +3302,21 @@ function Install-NodeDeps {
         [string]$exePath, [string]$argLine, [string]$workDir,
         [string]$logPath, [int]$timeoutSec
     ) {
+        # cmd.exe redirects the child's bytes into $logPath verbatim: no
+        # PowerShell decode happens on the way in, so the file carries the
+        # child's own encoding and no BOM.  Node-based children (npm, npx,
+        # playwright) write UTF-8.  Every Get-Content that reads this file
+        # therefore has to say -Encoding UTF8 -- without a BOM to sniff it
+        # would otherwise decode with the machine ANSI code page, which is
+        # the one thing the [Console]::OutputEncoding line near the top of
+        # this script cannot help with (that only fixes the display side).
         $cmdLine = "/d /s /c "" ""$exePath"" $argLine > ""$logPath"" 2>&1 """
         $proc = Start-Process -FilePath $env:ComSpec -ArgumentList $cmdLine `
             -WorkingDirectory $workDir -NoNewWindow -PassThru
         $deadline = [DateTime]::UtcNow.AddSeconds($timeoutSec)
         $shown = 0
         function _Drain-NewLines([string]$path, [ref]$count) {
-            $lines = @(Get-Content $path -ErrorAction SilentlyContinue)
+            $lines = @(Get-Content $path -Encoding UTF8 -ErrorAction SilentlyContinue)
             if ($lines.Count -gt $count.Value) {
                 $lines[$count.Value..($lines.Count - 1)] | ForEach-Object {
                     Write-Host "    $_" -ForegroundColor DarkGray
@@ -3364,7 +3375,7 @@ function Install-NodeDeps {
                 Write-Warn "$label npm install failed -- exit code $code"
             }
             if (Test-Path $logPath) {
-                $errText = (Get-Content $logPath -Raw -ErrorAction SilentlyContinue)
+                $errText = (Get-Content $logPath -Raw -Encoding UTF8 -ErrorAction SilentlyContinue)
                 if ($errText) {
                     $snippet = if ($errText.Length -gt 1200) { $errText.Substring(0, 1200) + "..." } else { $errText }
                     Write-Info "  npm output:"
@@ -3472,7 +3483,7 @@ function Install-NodeDeps {
                         Write-Warn "Playwright Chromium install failed -- exit code $pwCode"
                         Write-Warn "Browser tools will not work until Chromium is installed."
                         if (Test-Path $pwLog) {
-                            $pwErr = Get-Content $pwLog -Raw -ErrorAction SilentlyContinue
+                            $pwErr = Get-Content $pwLog -Raw -Encoding UTF8 -ErrorAction SilentlyContinue
                             if ($pwErr) {
                                 $snippet = if ($pwErr.Length -gt 1200) { $pwErr.Substring(0, 1200) + "..." } else { $pwErr }
                                 Write-Info "  playwright output:"

tests/test_install_ps1_native_log_encoding.py

@@ -0,0 +1,91 @@
+"""Regression tests: install.ps1 must state the codec when reading native logs.
+
+``_Invoke-NativeWithTimeout`` has cmd.exe redirect a child's stdout+stderr into
+a log file.  No PowerShell decode happens on the way in, so the file holds the
+child's own bytes and carries no BOM.  The children here are Node-based (npm,
+npx, playwright) and write UTF-8.
+
+Windows PowerShell 5.1's ``Get-Content`` sniffs a BOM and otherwise falls back
+to the machine ANSI code page, so a bare read of one of those logs decodes
+UTF-8 as cp932/cp936/cp1252.  Measured on ja-JP (ACP=932), PS 5.1.26100.9168::
+
+    no BOM  + bare Get-Content        -> error: 綢輔ぃ 經、綢ォ縺瑚九九▽縺九...
+    no BOM  + Get-Content -Encoding UTF8 -> error: ファイルが見つかりません
+    with BOM + bare Get-Content       -> error: ファイルが見つかりません
+
+The ``[Console]::OutputEncoding`` assignment near the top of install.ps1 does
+not cover this: its own comment says it is "a DISPLAY-only fix".
+
+Logs written through ``Tee-Object -FilePath`` are deliberately NOT in scope.
+PS 5.1 writes those as UTF-16LE **with** a BOM (measured: ``FF FE 42 30 ...``
+for a single "あ"), which Get-Content detects, so those reads are correct
+as they stand.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+INSTALL_PS1 = REPO_ROOT / "scripts" / "install.ps1"
+
+
+def _install_ps1() -> str:
+    return INSTALL_PS1.read_text(encoding="utf-8")
+
+
+# The four reads whose file is written by a native child, not by PowerShell.
+_NATIVE_LOG_READS = (
+    r"Get-Content\s+-LiteralPath\s+\$logPath\s+-Tail\s+\$TailLines[^\r\n]*",
+    r"Get-Content\s+\$path[^\r\n]*",
+    r"Get-Content\s+\$logPath\s+-Raw[^\r\n]*",
+    r"Get-Content\s+\$pwLog\s+-Raw[^\r\n]*",
+)
+
+
+def test_native_child_log_reads_state_the_codec() -> None:
+    text = _install_ps1()
+    for pattern in _NATIVE_LOG_READS:
+        matches = re.findall(pattern, text)
+        assert matches, f"expected a Get-Content matching {pattern!r}"
+        for line in matches:
+            assert "-Encoding" in line, (
+                "a log written by a native child has no BOM, so this read "
+                f"decodes with the machine ANSI code page: {line.strip()!r}"
+            )
+
+
+def test_cmd_redirect_helper_documents_why_the_codec_is_needed() -> None:
+    """The helper that creates the BOM-less logs explains the constraint."""
+    text = _install_ps1()
+    assert re.search(
+        r"cmd\.exe redirects the child's bytes[\s\S]{0,700}?"
+        r"\$cmdLine = \"/d /s /c ",
+        text,
+    ), "_Invoke-NativeWithTimeout must say why its log needs an explicit codec"
+
+
+def test_console_outputencoding_is_still_display_only() -> None:
+    """Pin the premise: the console fix does not reach Get-Content.
+
+    If a future change makes install.ps1 set a default read codec globally,
+    this test should be revisited rather than silently kept passing.
+    """
+    text = _install_ps1()
+    assert "[Console]::OutputEncoding = [System.Text.UTF8Encoding]::new()" in text
+    assert "DISPLAY-only fix" in text
+
+
+def test_tee_object_reads_are_left_alone() -> None:
+    """Tee-Object writes UTF-16LE with a BOM on 5.1; those reads are fine.
+
+    Kept as an explicit statement of scope so the next person does not
+    "finish the job" by adding -Encoding UTF8 to reads that would then
+    misdecode a UTF-16 file.
+    """
+    text = _install_ps1()
+    for var in ("$npmLog", "$buildLog"):
+        assert re.search(
+            r"Tee-Object\s+-FilePath\s+" + re.escape(var), text
+        ), f"{var} is expected to be written through Tee-Object -FilePath"