HDMI extended monitor has very low resolution in mirrored display mode

[Wang Xuancong]

It seems that recent versions of Ubuntu (16.04) has a bug in handling extended HDMI monitors. The HDMI monitor has up to 1980x1080 resolution, my laptop has up to 1366x768 resolution. Both screens have native 16:9 ratio.
However, in "mirrored display" mode, I can only select 1024x768 and 800x600 resolution as shown in the image attached.

In such mode, my laptop screen has two blank strip on the left and right side. On the HDMI monitor, it shows a squashed screen (1024x768 resolution stretched to 16:9 full screen).

I have temporarily developed a workaround, see https://xuancong84.blogspot.sg/2018/05/ubuntu-hdmi-extended-monitor-low.html , but I still hope the bug can be solved.

Thank you very much!

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: xorg 1:7.7+13ubuntu3
ProcVersionSignature: Ubuntu 4.15.0-15.16~16.04.1-generic 4.15.15
Uname: Linux 4.15.0-15-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.1-0ubuntu2.16
Architecture: amd64
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Tue May 8 22:21:37 2018
DistUpgraded: Fresh install
DistroCodename: xenial
DistroVariant: ubuntu
DkmsStatus: bcmwl, 6.30.223.271+bdcom, 4.10.0-28-generic, x86_64: installed
ExtraDebuggingInterest: Yes
GraphicsCard:
 Intel Corporation Device [8086:5916] (rev 02) (prog-if 00 [VGA controller])
   Subsystem: Dell Device [1028:07a0]
InstallationDate: Installed on 2018-03-09 (59 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
Lsusb:
 Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
 Bus 001 Device 003: ID 1bcf:2b96 Sunplus Innovation Technology Inc.
 Bus 001 Device 002: ID 17ef:6025 Lenovo ThinkPad Travel Mouse
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
MachineType: Dell Inc. Latitude 7480
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-15-generic root=UUID=3f217eea-d2a7-4c61-9f9a-0a111e2e3cdf ro quiet splash intel_pstate=disable vt.handoff=7
SourcePackage: xorg
Symptom: display
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/09/2018
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.9.3
dmi.board.name: 00F6D3
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.9.3:bd03/09/2018:svnDellInc.:pnLatitude7480:pvr:rvnDellInc.:rn00F6D3:rvrA00:cvnDellInc.:ct10:cvr:
dmi.product.family: Latitude
dmi.product.name: Latitude 7480
dmi.sys.vendor: Dell Inc.
version.compiz: compiz 1:0.9.12.3+16.04.20180221-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.83-1~16.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 17.2.8-0ubuntu0~16.04.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 17.2.8-0ubuntu0~16.04.1
version.xserver-xorg-core: xserver-xorg-core N/A
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati N/A
version.xserver-xorg-video-intel: xserver-xorg-video-intel N/A
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau N/A