Well, a little googling found a version of the TM74HC595Display.h library which has the .timerIsr() method. But nothing I can find online (in English) shows the .dispFloat() method. There may be a few pages in Russian which have that version of the library.
Additionally, the .timerIsr() method doesn't seem to be documented anywhere. Looking at the code, it looks like a "show" method which re-displays the current digits that the library has onto the display. Like maybe the display needs a constant refresh every millisecond or so?
I suspect that the reason for this causing problems is that the Dallas temperature sensor takes more than 1ms to read (up to one second if bus-powered) so the 1ms interrupts are disrupting communication with the sensor. This is a situation where it might make sense to detachInterrupt() for the period that the sensor is being read. Then attachInterrupt() again when finished. That may make the display go dark for 1 second. The actual time-critical parts of the sensor communication are shorter than 1ms, but you would have to hack your own version of the library to prevent that being interrupted by the display.