tk_messageBox does not display -detail argument correctly

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Version involved is:

% package require ActiveTcl
8.5.0.0.272633

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Actually, it's not so simple. The lengths of the "detail" that are displayed correctly and the lengths of the "detail" that have some garbage appended are intimately interspersed.

set detail ""
for {set i 0} {$i<60} {incr i} {
append detail [string index $i end]
tk_messageBox -message Count -detail $detail
}

I'm not even convinced it only depends on the length. But it's not right, anyhow.

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I see this on 8.5a6, Windows XP SP2 as well, compiled from source. Seems almost like it axes the terminating '\0' in the detail string before display.

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Update: this relates to win/tkWinDialog.c:2146. Tcl_DStringAppend() is given a length argument there, Tcl_DStringLength(&detailString). Changing this to "Tcl_DStringLength(&detailString)+1" APPEARS to fix the problem. (Using "-1" does not.) I'm not an expert regarding DStrings, 16-bit unicode, termination, and Windows peculiarities, but some interaction between these is at fault.

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Changed to code to avoid this problem and extended to test suite to check for this issue in the future.