fa6384eb39 introduced explicit setting of operator context as argument for
function `ED_region_panels_ex()`. However, this did not work correctly, because
`UiLayout` did not exist when `uiLayoutSetOperatorContext()` was called. This
has to be done in `ed_panel_draw()`.
Another issue is, that panel may be drawn, when mouse is over tool region,
which means, that `sequencer_tools_region_draw()` must look for whether
this is happening in preview or timeline region.
Pull Request: https://projects.blender.org/blender/blender/pulls/118292
a2c839e71c