Chrome Version: 58.0.3029.40 (Official Build) beta (32-bit)
OS: 9334.28.0 (Official Build) beta-channel kevin

What steps will reproduce the problem?
(1) Connect to HP ZR30w external monitor
(2) Switch chrome://settings/display to Mirror Mode

What is the expected result?

Both internal and external display will be driven with 2400x1600 phyiscal pixels in HighDPI (1200x800 logical pixels).

What happens instead?

The internal display runs in HighDPI as expected, but the external display seems to be driven with 1200x800 physical pixels and no HighDPI (just relying on the monitor's internal interpolation which looks awful).

Please use labels and text to provide additional information.

The machine is perfectly capable of driving the external monitor at native resolution... without Mirror Mode I get 2560x1600 physical pixels just fine. Due to the lack of progress on issue 442441 this used to be the last possible way we could get an external display into HighDPI mode, and now that no longer works either.

(On the side: the resolution drop-down is also grayed out while I am in mirror mode, even though the system is currently able to mirror at both 1200x800 and 2400x1600 as long as I set that resolution before mirroring.)

Some modetest output, let me know if you need more:

Encoders:
id	crtc	type	possible crtcs	possible clones	
31	25	TMDS	0x00000003	0x00000000
33	28	TMDS	0x00000002	0x00000000

Connectors:
id	encoder	status		name		size (mm)	modes	encoders
32	31	connected	eDP-1          	259x173		1	31
  modes:
	name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot)
  2400x1600 60 2400 2448 2480 2564 1600 1603 1613 1733 266667 flags: nhsync, nvsync; type: preferred, driver
  props:
	1 EDID:
		flags: immutable blob
		blobs:

		value:
	2 DPMS:
		flags: enum
		enums: On=0 Standby=1 Suspend=2 Off=3
		value: 0
34	33	connected	DP-1           	640x400		2	33
  modes:
	name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot)
  2560x1600 60 2560 2608 2640 2720 1600 1603 1609 1646 268500 flags: phsync, nvsync; type: preferred, driver
  1280x800 60 1280 1328 1360 1440 800 803 809 823 71000 flags: phsync, nvsync; type: driver
  props:
	1 EDID:
		flags: immutable blob
		blobs:

		value:
			00ffffffffffff0022f06c2801010101
			17160104b5402878e28d85ad4f35b125
			0e505400000001010101010101010101
			010101010101e26800a0a0402e603020
			360081902100001abc1b00a050201730
			3020360081902100001a000000fc0048
			50205a523330770a20202020000000ff
			00434e34323233304250330a20200050
	2 DPMS:
		flags: enum
		enums: On=0 Standby=1 Suspend=2 Off=3
		value: 0

CRTCs:
id	fb	pos	size
25	45	(0,0)	(2400x1600)
  2400x1600 60 2400 2448 2480 2564 1600 1603 1613 1733 266667 flags: nhsync, nvsync; type: preferred, driver
  props:
28	42	(0,0)	(1280x800)
  1280x800 60 1280 1328 1360 1440 800 803 809 823 71000 flags: phsync, nvsync; type: driver
  props:

Planes:
id	crtc	fb	CRTC x,y	x,y	gamma size	possible crtcs
23	25	45	0,0		0,0	0       	0x00000001
  formats: XR24:LINEAR AR24:LINEAR XB24:LINEAR AB24:LINEAR RG24:LINEAR BG24:LINEAR RG16:LINEAR BG16:LINEAR NV12:LINEAR NV16:LINEAR NV24:LINEAR
  props:
	5 type:
		flags: immutable enum
		enums: Overlay=0 Primary=1 Cursor=2
		value: 1
24	25	36	0,0		0,0	0       	0x00000001
  formats: XR24:LINEAR AR24:LINEAR XB24:LINEAR AB24:LINEAR RG24:LINEAR BG24:LINEAR RG16:LINEAR BG16:LINEAR
  props:
	5 type:
		flags: immutable enum
		enums: Overlay=0 Primary=1 Cursor=2
		value: 2
26	28	42	0,0		0,0	0       	0x00000002
  formats: XR24:CHROMEOS_ROCKCHIP_AFBC XR24:LINEAR AR24:CHROMEOS_ROCKCHIP_AFBC AR24:LINEAR XB24:CHROMEOS_ROCKCHIP_AFBC XB24:LINEAR AB24:CHROMEOS_ROCKCHIP_AFBC AB24:LINEAR RG24:LINEAR BG24:LINEAR RG16:LINEAR BG16:LINEAR NV12:LINEAR NV16:LINEAR NV24:LINEAR
  props:
	5 type:
		flags: immutable enum
		enums: Overlay=0 Primary=1 Cursor=2
		value: 1
27	0	0	0,0		0,0	0       	0x00000002
  formats: XR24:LINEAR AR24:LINEAR XB24:LINEAR AB24:LINEAR RG24:LINEAR BG24:LINEAR RG16:LINEAR BG16:LINEAR
  props:
	5 type:
		flags: immutable enum
		enums: Overlay=0 Primary=1 Cursor=2
		value: 2
29	0	0	0,0		0,0	0       	0x00000002
  formats: XR24:LINEAR AR24:LINEAR XB24:LINEAR AB24:LINEAR RG24:LINEAR BG24:LINEAR RG16:LINEAR BG16:LINEAR NV12:LINEAR NV16:LINEAR NV24:LINEAR
  props:
	5 type:
		flags: immutable enum
		enums: Overlay=0 Primary=1 Cursor=2
		value: 0
30	0	0	0,0		0,0	0       	0x00000002
  formats: XR24:LINEAR AR24:LINEAR XB24:LINEAR AB24:LINEAR RG24:LINEAR BG24:LINEAR RG16:LINEAR BG16:LINEAR
  props:
	5 type:
		flags: immutable enum
		enums: Overlay=0 Primary=1 Cursor=2
		value: 0