FWIW, if I add this after the two calls to SetActive, it works
m_panel.enabled = false;
m_panel.enabled = true;
oh, and m_panel.Invalidate(false) does *not* work, just doing that enable toggling
EDIT:
m_panel.Refresh() works too.
Huh, never needed to do that. I guess that was from the perf changes?
EDIT 2:
Going back to the UIPopupList occurrence,
Calling mPanel.Refresh() inside UIPopupList.OnClick doesn't seem to fix that. Manually togging the panel disabled/enabled in the editor does fix the UIPopupList (until it's opened again).