Editor scrolls back to top when returning to it after Quick Open

[peterflynn]

Finally nailed down some repro steps for this!

1. Open file A, scroll down one or more screenfuls and place the cursor somewhere there
2. Use Quick Open to open some other file B
3. Switch back to file A (via whatever mechanism -- Ctrl+Tab, click in working set, etc.)

[pthiess]

Reviewed, medium priority seems reasonable, @njx

[peterflynn]

Looks somewhat unrelated to #2985 after all. I think the bug here is that the value of getCurrentFullEditor() during the ModalBar ctor isn't necessarily the same as the value of getCurrentFullEditor() during ModalBar.close(). Because closing happens at the end of QuickOpen._handleItemSelect() and is further delayed by a timeout, it's basically guaranteed that the editor will have already switched by the time close() is called.

Things brings two questions to mind:

• Is it even possible to restore the scroll pos of the correct editor after it's invisible? Will CM choke trying to measure something in code that was triggered by scrollTo()? Will the update stick once the editor becomes visible and gets refreshed?
• What about the editor you were switching to? If it's also affected by the ModalBar's presence (before it closes), does that mean we have to run the restore-scroll-pos code on both editors?

[peterflynn]

Also note: the timing of this stuff is affected by whether file B is already in the working set -- the editor switch is much more synchronous if so.

[dangoor]

I can still reliably reproduce this after the landing of #3023, so I guess you're right about it not being quite related.

[RaymondLim]

@dangoor I believe the root cause of this is also causing your Quick Open unit test failing (#2696).

[njx]

Nominating for sprint 22.

[njx]

Turns out this is due to a subtle race condition: if you call scrollTo() on a CodeMirror instance and then immediately hide the editor before the resulting scroll events are processed, CM ends up scrolling to (0,0). I submitted codemirror/codemirror5#1350 to fix this.

[njx]

FBNC to @peterflynn

[peterflynn]

Confirmed