Hi Sonny, would you please add more contexts about this issue? For example, 
- What device type, audio or HCI types, is used in your test? 
- Is it BR/EDR or LE? 
- And what is impact of this symptom?

The following is just for the record.
I conducted the test with a LE mouse. The scanning operations look like
- scanning is going on for 10 seconds
- scanning is paused for 4 seconds
And the above operations repeat. During the pause period, the connection could be made.

Hi Joseph,

Here are more information:
- The device type is BREDR, tested with headphones.
- The scanning that affects the issue can be BREDR, LE, or dual.
- The impact is that user cannot connect to headphones while bluez is doing scanning.
- This is confirmed to be reproducible in eve and chell, and confirmed to be not reproducible in peppy and minnie. Looks like there is correlation with Intel chip. (Also reproducible in a non-chromebook laptop running Ubuntu 16.04 using Intel BT chip).

Thanks Sonny for the information! Is this reproducible when using Chrome UI to connect to a device?

Hi Joseph,

Yes, it's reproducible with Chrome UI. But the problem with Chrome UI is that you can't unreproduce it, i.e. you can't stop scanning if you are using Chrome UI. When you open the system tray or chrome://settings Bluetooth page, scanning is automatically started, if you wait a few seconds scanning will eventually stop and then you can connect to headphones.

we have similar open issue

https://bugs.chromium.org/p/chromium/issues/detail?id=523176