See the discussions in https://github.com/adobe/brackets/pull/7693.

`calc(100vh - 34px)` doesn't work in LESS because it tries to calculate the value at **compile** time (not **run** time).

The quotes are supposed to tell LESS to pass it through. I also tried in the LESS 1.7 branch (https://github.com/adobe/brackets/pull/6730), but it didn't work there either.