The following problem exists in the current git version, but is not new.
When a new dockable dialog is created, for example by double-clicking a tool
icon in the Toolbox when there is no Tool options dialog open, or by
double-clicking a paint dynamics entry in the Paint dynamics dialog, the new
dialog is open under the current window. Sometimes, it is almost completely
hidden. Either it should be open over the current window, or in another
location on the screen, if available.
Gimp-developer mailing list