Cygwin/X with Win10 display scaling corrupting font display of typed characters - Issue identified

[Jon Turney]

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

[...]
>>> If I change the scaling from 125% to 100% on the laptop's display,
>>> the problem appears until I restart Cygwin/X. Restarting Cygwin/X
>>> shows it displaying properly, until I change the scaling again.
>>
>> I could only reproduce this problem with mis-rendering when changing 
>> the scaling on the secondary monitor.
> 
> Wow, I did a really poor job of writing that. I'm sorry.
> 
> For clarity, just in case you were unable to interpret what I meant by 
> what I wrote -
> 
> At start: Laptop scaling set at 125%, second monitor at 100%.
> 
> Mis-rendering occurs at start, on the second monitor only.
> 
> If I change the scaling on the laptop, while the current instance of 
> XWin is running - the same mis-rendering now occurs on the laptop.
> 
> Interestingly enough, if I change the laptop from 125% to 100%, the tops 
> are clipped as previously reported. But if I change the scaling from 
> 125% to 150%, then the bottoms are clipped. (It kinda makes sense from 
> what you've written.)
> 
> If I then stop and restart XWin after having reset the scaling such that 
> both monitors have the same setting, then the problem doesn't appear.

Thanks for the clarification.

The laptop display is the primary monitor in all cases, correct?