In generic/tclNotify.c, in the function Tcl_SetMaxBlockTime, checking for tsdPtr->blockTimeSet in the second if-clause is unnecessary. The current code looks as follows:

if (!tsdPtr->blockTimeSet || ...) {
    ...
    tsdPtr->blockTimeSet = 1;
}

if (!tsdPtr->inTraversal) {
    if (tsdPtr->blockTimeSet) {
        Tcl_SetTimer(&tsdPtr->blockTime);
    } else {
        Tcl_SetTimer(NULL);
    }
}

Either tsdPtr->blockTimeSet is already 1 when we enter this function, or otherwise the first if-clause will set it to 1. Therefore, the second if-clause can simply read

if (!tsdPtr->inTraversal) {
    Tcl_SetTimer(&tsdPtr->blockTime);
}