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Message-ID:	<b5d8769c-cf7c-e654-83c0-b7f375e5f64b@dronecode.org.uk>
Date:	Sat, 5 Feb 2022 14:25:48 +0000
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Subject:	Re: Cygwin/X with Win10 display scaling corrupting font display of
	typed characters - Issue identified - "Solution" found
To:	Ken Whitesell <KenWhitesell AT comcast DOT net>,
	The Cygwin Mailing List <cygwin AT cygwin DOT com>
References:	<b56c0a7e-27f3-c165-6331-69b71b2075a5 AT comcast DOT net>
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	<4f4d8d34-3e30-82fe-4be2-2926da2c1c6a AT dronecode DOT org DOT uk>
	<ebef1f87-1457-3068-2657-af5dfa5c484e AT comcast DOT net>
	<37ce4364-d009-4280-ebf2-739b5aa6a66d AT dronecode DOT org DOT uk>
	<2e8f1d16-92eb-b66e-c49e-7e38e5f07110 AT comcast DOT net>
From:	Jon Turney <jon DOT turney AT dronecode DOT org DOT uk>
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On 27/01/2022 03:12, Ken Whitesell wrote:
> On 1/24/2022 10:02 AM, Jon Turney wrote:
>> On 20/01/2022 01:01, Ken Whitesell wrote:
>>> On 1/19/2022 2:28 PM, Jon Turney wrote:
>>>> On 19/01/2022 00:02, Ken Whitesell wrote:
>>>>> On 1/17/2022 1:29 PM, Ken Whitesell wrote:
>>>>>>
>>>>>> Is there a known solution for this? (Or is it known that there is 
>>>>>> no solution?)
>>>>
>>>> Thanks for reporting this.
>>>>
>>>>>> Any guidance, pointers, suggestions of avenues for further 
>>>>>> research, or other information, will all be greatly appreciated.
>>>>>>
>>>>> After more research and experimentation, it appears to be related 
>>>>> to one of xorg-server, xorg-server-common, or xorg-server-xorg.
>>>>>
>>>>> Installing the older version 1.20.12-1 of these packages allows the 
>>>>> windows to be moved between monitors without any issues. Upgrading 
>>>>> to the current version 21.1.3-1 creates the problems. I'm able to 
>>>>> replicate this behavior on two different laptops with two different 
>>>>> external monitors.
>>>>
>>>> It seems likely that this is an unintended effect of changes in 
>>>> xorg-server 21.1.0-1, trying to fix problems in this area (See [1])
>>>
>>> Thanks for the references. I've read all the messages in the thread - 
>>> I was particularly intrigued by this comment:
>>>
>>> wrt the font scaling issue, looking at the source, it seems that we
>>> don't re-consider the display dpi after a WM_DISPLAYCHANGE message, but
>>> keep on using the value determined at startup.  This is probably a bug.
>>>
>>> I'm curious enough to want to take a look at the code, but I've got 
>>> no belief that I'm going to be able to find an answer. (I'm *not* a 
>>> C++ programmer. I can read it and write a little of it, but that's 
>>> about it.) I was going to start by comparing the last known-working 
>>> version to the first known-non-working version, but given that it's a 
>>> major release change, that's not likely going to be a useful 
>>> approach. (I'm way out of my league here. It's probably going to take 
>>> me a long time just to get to the point where I can even begin to 
>>> explore this.)
>>
>> The relevant change, which tries to fix the issue identified in that 
>> comment, and probably introduces this issue is:
>>
>> https://gitlab.freedesktop.org/jturney/xserver/-/commit/b19b6266d33f2b911dc1826ad5c03da135a39957 
>>
>>
>> [...]
[...]
> 
> But, the real reason for my reply at this point is to report that I have 
> found a solution for _my_ issue.
> 
> I make no guarantees about any problems anyone else may be facing, nor 
> can I make any statement about whether or not this causes other problems.
> 
> Obligatory disclaimer: I don't really know what I'm doing here. I'm not 
> a Windows developer, and I know just enough about cygwin to muddle my 
> way through doing what I want to do with the help of the mailing lists 
> and other resources. I'm not in a position to _explain_ anything. This 
> works for me, but that's as far as I can go.
> 
> First, the bottom line:
> 
> XWin.exe.manifest, line 21
> 
> change:
> <dpiAwareness>PerMonitorV2,PerMonitor</dpiAwareness>
> to
> <dpiAwareness>PerMonitor</dpiAwareness>
> 
> Some details:
> 
> I managed to get to a point where I could build the packages from source 
> and install them. I looked at the commit you referred me to, and started 
> reverting changes, one-by-one - at least in so far as the change 
> appeared to make sense to me.
> 
> Anyway, I got to this change, and sure enough, it worked. Removing the 
> "PerMonitorV2" solved the issue. Also, I confirmed that it's the 
> "PerMonitorV2" that is causing the issue and not having both of them by 
> running another test with just the "PerMonitorV2" - and that still shows 
> the problem.

Thanks for taking the time to narrow this down.  It's been very helpful.

Working through the documented effects of that [1], I was able to work 
out that this mis-rendering is due to the non-client area scaling.

[1] See under DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 in
https://docs.microsoft.com/en-us/windows/win32/hidpi/dpi-awareness-context

If you turn off 'Hide Root Window' from the tray menu, you can kind of 
see what's going wrong: normally the client area of the X window is 
exactly aligned with that in the root window, but when the non-client 
area is rescaled by a DPI change (the title bar changes size 
significantly), it's misaligned so that part of the X window is outside 
the client area (and thus clipped in updates).

I think this is due to the AdjustWindowsRectEx() function not being DPI 
aware (I guess it's always computing the non-client window rect based on 
the processes initial DPI)

Unfortunately, from an initial look, rewriting things to use 
AdjustWindowRectExForDpi() isn't trivial (since we need to make 'DPI of 
the monitor this Window is going to end up on' available to it)

So for the moment, I think I'll apply your reversion (although this 
probably comes at the cost of not scaling the window frame, the traymenu 
and About... dialog)


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