Whenever I run `dmesg' on a xterm that uses the following options:

xterm -fa "DejaVu Sans Mono:style=Book:antialias=true" -fs 9

and I scroll up with the keyboard (shift+page up) parts of the scrollback
buffer disappears, making it difficult to read the output.

I've tested other terminals (urxvt and alacritty) and I cannot trigger the bug
with these terminals, with xterm it happens only with xft and with the options
I mentioned above.

Steps to reproduce:

- run `xterm -fa "DejaVu Sans Mono:style=Book:antialias=true" -fs 9'
- run `dmesg', hit enter and page up

You should notice parts of the buffer disappearing.

I use the modesetting DDX driver, I also tried the xf86-video-amdgpu DDX driver
and I can reproduce it with both, however, when setting Option "Accel" "off" in
/etc/X11/xorg.conf.d/20-amdgpu.conf I cannot reproduce it anymore.

Actual Results:

As mentioned, the scrollback buffer disappears when paging up with these xterm
options that I mentioned above.

Expected Results:

xterm works normally with these options, with no scrollback buffer issues.

System details:

- Arch Linux (x86_64)
- linux 5.0.2-arch1-1-ARCH
- xorg-server 1.20.4-1
- i3-wm 4.16.1-1
- xterm 344-1
- mesa 19.0.0-1

My machine is a "AMD Ryzen 5 2400G with Radeon Vega Graphics" APU with
integrated vega graphics.

$ lspci | grep VGA
07:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Raven
Ridge [Radeon Vega Series / Radeon Vega Mobile Series] (rev c6)