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I have GNU screen configured to use Ctrl+Space as the escape character. This means that Ctrl+SpaceSpace will send a literal Ctrl+Space to the process in the window. Also, Ctrl+Space Ctrl+Space is the mapping for "switch to next window."

So now I have a conundrum. When I hold down Ctrl and double tap space, that should be doing Ctrl+Space Ctrl+Space, and switching to the next window. Sometimes it does, and sometimes it doesn't.

When I fire up showkey -a to see what the window is receiving, when I type Ctrl+Space Ctrl+Space quickly, the result seems to be ^[ (the escape character). But, it's also sent a Ctrl+Space to screen itself, so that the next character I type is taken as screen command—making for extreme confusion.

Worst of all, it is not deterministically reproducible. Sometimes it works consistently (i.e. Ctrl+Space Ctrl+Space will switch windows no matter how fast I type it), and sometimes I have to type it slow or I get the behaviour described above (consistent for several minutes).

What could be causing Ctrl+Space Ctrl+Space to instead be sending Escape Ctrl+Space?

I am using MobaXterm on Windows 10, but GNU screen is actually running on a RHEL 7.5 host that I am logged into via SSH.

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  • There's no such thing as a ^SPC character. What character (or sequence of character) does your terminal emulator send upon Ctrl+Space (press Ctrl+V Ctrl+Space outside of screen)? What value to you use as the escape setting in screen? Commented Aug 24, 2019 at 7:06
  • @StéphaneChazelas In terminals Ctrl-Space typically generates the NUL byte. Not sure about MobaXterm, though. Commented Aug 24, 2019 at 8:16
  • I thought CTRL-@ 0x00 was infact null, it comes before CTRL-A 0x01, etc - It was emacs that interprets ^@ as equal ^-SPC for the binding to set-mark, and it's grown from there and I believe @Stéphane Chazelas is correct CTRL-SPC isn't actually an ascii character, but ^-@ ^-A, ^-B, ... was used to be able to enter 0x00-0x1F on an ascii keyboard. @ = 0x40 A = 0x41, Commented Aug 24, 2019 at 8:48
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    Yes, ^@ is the usual "control" representation of NUL and it's true many terminals send NUL/^@ upon Ctrl+Space, Ctrl+@, Ctrl+Backtick, where the Ctrl modifier takes the lowest five bits of the character that would be sent without Ctrl. screen for its escape command seems to interpret "^ " the same as "^@" and mean NUL, so it's likely what the OP used, and what the OP's terminal sends. Commented Aug 24, 2019 at 8:58
  • When you type Ctrl+Space in sequence outside of screen, do you always see the same characters (presumablly ^@/NUL)? Commented Aug 24, 2019 at 9:10

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