In bug 1589102, changes were made to how about:blank and about:srcdoc load,
making them go via DocumentChannel and thus slightly increasing documents' load
times. This causes some timing issues in SessionStore code.
When we click to "Reopen all (closed) Windows", undoCloseWindow() is called for
each window that we have to restore. The following scenario can occur - we are
restoring window 1, and undoCloseWindow() has already called restoreWindows,
after receiving the window showing promise, and in the meantime
undoCloseWindow() has been called for window 2, and there is now a promise in
the WINDOW_SHOWING_PROMISES. But now restoreWindows() for window1 calls
_openWindows(), where root.windows length is 0, but WINDOW_SHOWING_PROMISES is
not empty (because of window 2's undoCloseWindow), and the resolve callback for
_openWindows() gets called with wrong arg. To solve this, ensure that the
promises returned from _openWindows() correspond to the windows opened
within the body of the function.
Differential Revision: https://phabricator.services.mozilla.com/D106304
c52cdbd749