It's been a while, but looking at the code, I believe it is to keep
track of the mouse mode when the toolbar is hidden. That is: when you
hide the toolbar, you want to revert to NoMode as it could be awkward to
have some mouse mode enabled, while not being able to change it; so
m_mouseMode is reset to NoMode, as it's being used in the rest of the
code. However, when you show the toolbar again, you may want to restore
the mouse mode to what it was initially (especially to keep the visual
state of the toggle buttons in sync with the actual mouse mode).
It's probably better to check first in Matlab how this is handled. Maybe
I made things to complicated and we should just maintain the mouse mode,
even after hiding the toolbar.