fix: preserve screenshot fallback encoding errors

Co-authored-by: Orca <help@stably.ai>

src/main/browser/cdp-screenshot.test.ts

@@ -190,6 +190,28 @@ describe('captureScreenshot', () => {
     )
   })
 
+  it('reports the original timeout when fallback encoding fails', async () => {
+    vi.useFakeTimers()
+
+    const webContents = createMockWebContents()
+    webContents.debugger.sendCommand.mockImplementation(() => new Promise(() => {}))
+    webContents.capturePage.mockResolvedValueOnce({
+      isEmpty: () => {
+        throw new Error('native image unavailable')
+      }
+    })
+    const onResult = vi.fn()
+    const onError = vi.fn()
+
+    captureScreenshot(webContents as never, { format: 'png' }, onResult, onError)
+    await vi.advanceTimersByTimeAsync(8000)
+
+    expect(onResult).not.toHaveBeenCalled()
+    expect(onError).toHaveBeenCalledWith(
+      'Screenshot timed out — the browser tab may not be visible or the window may not have focus.'
+    )
+  })
+
   it('reports the timeout when both CDP and fallback capture stall', async () => {
     vi.useFakeTimers()
 

src/main/browser/cdp-screenshot.ts

@@ -268,7 +268,13 @@ export function captureScreenshot(
             clearTimeout(fallbackTimer)
             fallbackTimer = null
           }
-          const fallback = encodeNativeImageScreenshot(image, params)
+          let fallback: { data: string } | null = null
+          try {
+            fallback = encodeNativeImageScreenshot(image, params)
+          } catch {
+            settleError(SCREENSHOT_TIMEOUT_MESSAGE)
+            return
+          }
           if (fallback) {
             settleResult(fallback)
             return