There are various long standing problems with 'refreshable' widgets (refreshed using refresh-now), which makes it extremely troublesome to develop new non trivial widgets for TeXmacs.
The precise problems tend to depend on the specific version of Qt being used, but as a general rule, refreshable widgets are nearly impossible to use at present.

In order to reproduce one typical such bug, uncomment the 'refresh-now' statement in 'cell-set' in 'table-widgets.scm'.  Next, create a table, open Format -> Cell, and change various cell properties.  Depending on the OS/Qt-version, the cell property window disappears, is not refreshed appropriately, or is assigned an invalid size.

Besides, in case of changes inside a widget, the size of the enclosing window is often recomputed in an incorrect manner.  This is also a very annoying bug for the development of TeXmacs widgets.