Long lines in the declared premises on tree creation are cut off

[lf-]

This came from this exercise I grabbed from the forallx-UBC textbook while writing docs:

```{.TruthTree .Prop}
1.1 A&B,~(C\/D),(~B\/C)\/E,~E
```

This happened with Firefox 89 on Linux. It also happened with Chromium 91, also on Linux.

Note that the "declare premises" button fixes this issue: this only happens when createTree is called with a tree with long parts and "declare premises" is not pressed again.