flags doesn't pass CV_EVENT_FLAG_LBUTTON / CV_EVENT_FLAG_MBUTTON / CV_EVENT_FLAG_RBUTTON press modifiers during the mouse movement.

Test example: samples/python/find_obj.py

broken since #20253

Reference: https://docs.gtk.org/gtk3/func.accelerator_get_default_mod_mask.html

The modifier mask determines which modifiers are considered significant for keyboard accelerators.