While using the preview functionality (Alt+P), the syntax highlighting of the previewed source vanishes, except for some small parts around the last edit.
I already played with the priorities in the syntax highlighting menu (setting preview to higher or lower priority compared to the basic highlighting), but this didn't help.
TeXstudio 2.9.4 (hg 5006:b43062855b2d)
Using Qt Version 5.4.2, compiled with Qt 5.4.1 R
works fine here.
Have you set a text color for the preview-highlighting.
It should only have a background ...
Assuming that right-clicking disables a color, I don't have a foreground color set.
I append three pictures: Before enabling the preview, immediately after pressing Alt+P and after adding and removing a space after the first formula.
fixed in repository