ConEmu build: 170605 alpha x64
OS version: Windows 10 1703 x64 (build 15063.413)
Used shell version (Far Manager, git-bash, cmd, powershell, cygwin, whatever): cmd
When resizing the terminal window to a size smaller than the width of the output, the output disappears entirely. Seems like an issue with ConEmu, as cmd is working fine.
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In case it helps, here is my settings of the cmd window. If I recall correctly, I've installed in the past the WSLBridge, before upgrading to Creators Update.

This looks like a conhost bug. I've seen that several times, but without 100% repro.
You may try to turn "legacy console mode" on.
/cc @miniksa
Tried it with legacy console mode on, the text doesn't disappear this time, but it's cropped after resizing the window back. cmd still works fine (without wrapping though).

Yep, that's expected...
I consider this as external issue. Regardless the fact you can't reproduce it in bare conhost, because different API are used. ConEmu is allowed to change console buffer size programmatically only.
I've filed MSFT:12339782 to take a look at this when I get a chance to see if there is anything I can do.
@miniksa Just reproduced. And I realized that contents did not disappear. In fact, text foreground color was set to the same value as background.
So, if you try to select text in ConEmu you'll see hidden contents. RealConsole looks like empty one, even after text selection, but Ctrl+C do copy hidden text too.
Did a little bit of digging around. There was an issue MSFT: 11561936 where resizing the console window smaller while in traditional mode (word wrap/resize is set to OFF) would lose some of the color attributes. I fixed that one a few weeks ago and it is on its way out to Insiders.
Seems to be fixed in Insider build 16237, closing

Same problem for me on Windows 10 x64 even in preview versions.

@asolopovas Have you checked WhatsNew??
https://conemu.github.io/blog/2017/08/19/Build-170819.html
Most helpful comment
Did a little bit of digging around. There was an issue MSFT: 11561936 where resizing the console window smaller while in traditional mode (word wrap/resize is set to OFF) would lose some of the color attributes. I fixed that one a few weeks ago and it is on its way out to Insiders.