Text is always "on top"... unless...
When dealing with text in the Viewer AND ANYTHING that has control points (like OnScreen Controls... etc.,), you have to change the tool you're dealing with. [The only other tool that changes the cursor in the Viewer is the Hand tool.]
The tools you use in the storyline can be activated in the viewer (the Viewer pane needs to be the active pane). By default, the Select Tool (A) is the tool that is active in the Viewer. You deal with Text with the Select Tool.
When you have Text and you need to access an OSC (or other control point to another object) you can type the B key (Blade), T (Trim), P (Position), R (Range) or the H key (Hand) and you will have CLEAR access to any of the OSCs or control points that may be over the text area. When the Select tool (A) is active, Text has the "focus" like it's on top of everything else (layer wise). You can switch back and forth between tools as you work in the Viewer (by either the tool menu or by using the keyboard shortcuts).
You can tell the region that Text controls by clicking on the text in the viewer. You will see a bounding box. If all the control points of the other object (like your mask) are outside the text bounding rectangle, you don't need to change the tool — they'll be accessible. Only when the control points you need to access are inside that rectangle.
You may need to get used to typing off Select (B, T, R, P, H) and back (A) to manage multiple objects with text in the Viewer, especially when you click back out of the Viewer into the storyline.
The one other tool that is available is Zoom (Z). However, it does not work for control points (because it will actually zoom the viewer).
HTH
