Pulling a panel's grip into the corner of the screen pushed it a couple of
pixels past the main window, and ImGui gave it an OS window of its own. Now
a panel only comes out when dragged with the cursor outside the main window.
A panel that stays in the main window needs its grips in reach, so a resize
stops at the edge, and a panel left past the edge or too big for the window
is moved back and shrunk to fit.
This is a new io.ConfigViewportsNoAutoDetach option in our copy of ImGui, so
it has to be carried over when ImGui gets updated. ImGui itself has no option
for this, as of 1.93.1.