Steps to reproduce:

1. Start Nightly
   --- enlarged
2. And then、Click [+] button
   --- not enlarged, but Addressbar still get focus
3. And then. Click empty area of page, then Click [+] button
   --- enlarged
4. And then、Click [+] button
   --- not enlarged (same as step2)
5. Switch to other application by cliking taskbar button and then back to Nightly
   --- enlarged
6. And then. Click empty area of page, then Middle click [Home] button
   --- not enlarged
7. Click [Home] button
   --- enlarged
8. Click [Reload] button
   --- not enlarged
9. Click [Home] button
   --- not enlarged

Actual Results:
see the above str

Expected results:
I think it should be consistent,

(In reply to Alice0775 White from comment #0)

2. And then、Click [+] button
   --- not enlarged, but Addressbar still get focus
3. And then、Click [+] button
   --- not enlarged (same as step2)

cannot reproduce

5. Switch to other application by cliking taskbar button and then back to Nightly
   --- enlarged

This actually depends on where you click, if you click the taskbar icon, or content or the toolbar.
I think the enlarged behavior is correct here, so not a bug.

6. And then. Click empty area of page, then Middle click [Home] button
   --- not enlarged

this looks like a bug, middle clicking home opens a new tab to about:newTab, it should enlarge.

8. Click [Reload] button
   --- not enlarged

this also looks like a bug

9. Click [Home] button
   --- not enlarged

as well as this one

It all depends on the decision about focus and collapsed state, so not worth looking into this until that is done.

back to p3 since we are fixing bug 1589826

Bug 1589826 fixes all the issues listed in the description. The Megabar remains consistently enlarged.