Fix HID read busy-spin: pass 250ms, not 250ns, to ReadWithTimeout

go-hid's ReadWithTimeout takes a time.Duration, so the bare 250 was
250ns, which truncates to a 0 ms hid_read_timeout. On both Linux
(poll) and macOS (cond_timedwait) 0 ms means non-blocking, so the
button loop spun continuously: ~1.2 cores and ~7.5k context
switches/sec at idle, from startup.

Pass 250*time.Millisecond and match hid.ErrTimeout with errors.Is
ahead of the string fallback. Idle CPU drops to ~0%.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yH8jHn1SNzNnL58CRwTi9

internal/device/streamdeck.go

@@ -3,6 +3,7 @@ package device
 import (
 	"bytes"
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"image"
 	"image/jpeg"
@@ -10,6 +11,7 @@ import (
 	"runtime"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/sstallion/go-hid"
 	"golang.org/x/image/draw"
@@ -167,8 +169,13 @@ func (sd *StreamDeck) ClearKey(keyIndex int) error {
 // Returns (nil, nil) on timeout — callers should check context and retry.
 func (sd *StreamDeck) ReadButtons() ([]bool, error) {
 	data := make([]byte, readReportSize)
-	n, err := sd.dev.ReadWithTimeout(data, 250)
+	// ReadWithTimeout takes a time.Duration: a bare 250 would be 250ns, which
+	// truncates to a 0 ms (non-blocking) hid_read_timeout and busy-spins the loop.
+	n, err := sd.dev.ReadWithTimeout(data, 250*time.Millisecond)
 	if err != nil {
+		if errors.Is(err, hid.ErrTimeout) {
+			return nil, nil
+		}
 		// Linux hidraw returns errors (not (0,nil)) for non-fatal conditions:
 		// timeout waiting for data, or EINTR (signal interrupted).
 		// macOS IOHIDManager usually returns (0, nil) on timeout, but may also

mac-support.md

@@ -96,3 +96,4 @@ The biggest architectural difference is privileged commands:
 | Log location | `journalctl --user -u streamdeck-go` | `~/Library/Logs/streamdeck-go.log` |
 | Config path | `~/.config/streamdeck-go/` (XDG) | `~/.config/streamdeck-go/` (XDG — works fine on macOS for CLI tools) |
 | Sleep/wake | Handled by reconnect loop | Handled by reconnect loop (same code) |
+| HID read timeout | `hid_read_timeout` → `poll()`; 0 ms = non-blocking | `hid_read_timeout` → `pthread_cond_timedwait`; 0 ms = non-blocking. Same fix applies: pass `250*time.Millisecond` |