Web Inspector: REGRESSION: Resources: WI.TreeOutline assertions when refreshing the page

[Devin Rousso]

# Steps to reproduce:
1. inspect <https://apple.com>
2. select the <https://apple.com> frame tree element in the Resources tab
3. refresh the page
 => many assertions fire
     - Assertion Failed: Unable to get index for tree element.
     - Assertion Failed: Index must be a non-negative integer.

[Radar WebKit Bug Importer]

<rdar://problem/47802027>

[Devin Rousso]

It looks like `WI.TreeOutline`'s `_cachedNumberOfDescendents` is a negative value.

[Matt Baker]

Did some digging. When the frame navigates, FrameTreeElement calls TreeElement.prototype.removeChildren, which the immediate base class, FolderizedTreeElement overrides:

removeChildren()
{
    super.removeChildren();

    this._clearNewChildQueue();

    for (var folder of this._folderTypeMap.values())
        folder.removeChildren();

    this._folderExpandedSettingMap.clear();
    this._folderTypeMap.clear();

    this._groupedIntoFolders = false;
}

Calling `removeChildren` for each FolderTreeElement leads to the assertions when trying to calculate the indexes of TreeElements which are no longer attached the tree.

I think the fix is to simply remove this step. I don't think it is necessary. We clear the map below, and I don't believe we hold on to any references to the FolderTreeElements after this function returns.

[Matt Baker]

Created attachment 361124 [details]
Patch

[Joseph Pecoraro]

Comment on attachment 361124 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=361124&action=review

> Source/WebInspectorUI/ChangeLog:12
> +        Calling `removeChildren` for deteached TreeElements is unnecessary.

Typo: deteached => detached

[Matt Baker]

Created attachment 361126 [details]
Patch for landing

[WebKit Commit Bot]

Comment on attachment 361126 [details]
Patch for landing

Clearing flags on attachment: 361126

Committed r240950: <https://trac.webkit.org/changeset/240950>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.