This is intentional. This circumflex is produced by a composed sequence, which is defined something like this:

<dead_circumflex> <dead_circumflex> : "^" asciicircum

In general the "~" there can be anything, e.g. "hello world". It's a different mechanism from the usual keysym->string translation.

In any case this bug belongs in libX11 not xkeyboard-config.

Here are some details on your trace if you're interested:

The ctrl xor is normally done by XkbTranslateKeySym(), which is called by XLookupString and also by XmbLookupString for "throughed" (non-filtered, unaltered, not part of a compose sequence) events. 

When you do <dead_circumflex><dead_circumflex>, the key events are processed by Xlib's Compose mechanism up until the second key press, which is filtered. This second key press event is redelivered to the client, but altered in two ways:

1. The keycode is changed to the special keycode 0.
2. Some metadata is set to indicate a complete sequence was composed.

When you call XLookupString on this event, you always get NoSymbol since the keycode is 0. XLookupString doesn't know anything about Compose or dead keys.

When you call XmbLookupString (which is Compose-aware) on this event, you get the composed asciicircum keysym and the "^" string as specified. In this code path XkbTranslateKeySym() is not called.

[Note that the "^" part in the compose sequence definition can be omitted. In this case the asciicircum string representation is used, but this is done ahead of time.]

However, if you *really* want the ctrl-xor behavior, you can simulate it with this snippet in your ~/.XCompose file:

include "%L"
<dead_circumflex> Ctrl <dead_circumflex> : "\x1e" asciicircum

Thanks a lot for the detailed explanation of what is happening here. As
this belongs to libX11, I suppose I should assign this bug to a
different "product"? Which one?

(Also, thanks for the workaround, it works nicely!)

I've tried to use the ~/.XCompose out of curiosity.

I am normally using `setxkbmap -layout de -variant nodeadkeys` where this is not a problem, but then tried it using `setxkbmap -layout de`, where it does not appear to make a difference, i.e. I cannot switch between tabs in Firefox.

I have used `im-config` to use "xim" as input method ($GTK_IM_MODULE=xim).

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