As part of the architectural efforts to remove XUL from mozilla-central, we are looking into deleting the "tree" widget entirely. While this removal will not happen very soon, the eventual plan of record does not involve developing a replacement that provides the same user experience. Instead, each of the current uses of "tree" in mozilla-central will be evaluated independently and replaced with an alternative that works for the specific use case. The consumers that use the most features of "tree" in mozilla-central are the Library window and the sidebar of Firefox. There are ongoing investigations to move these to a different user interface model. This would be a major project, and once it is completed, it may get rid the most complex dependency we have for removing "tree" from mozilla-central. In the meantime, we will start by removing other uses to see if there is anything obvious we may have missed in the analysis of the current consumers.
Does Thunderbird have a plan to replace its tree usage?
We don't know at the moment, there is bug 1446341 on file. The removal of "tree" will not happen very soon, and it may occur shortly before we remove all of XUL from mozilla-central. At that point, we will have a clearer picture of whether Thunderbird plans to migrate away from XUL entirely or adopt some of its features.
XUL <tree> is completely central to Thunderbird's UI. The XUL <tree> was originally implemented specifically for Thunderbird, and with the needs of Thunderbird in mind. The folder pane and the thread pane are XUL <tree>s, and the thread pane is the most prominent UI of Thunderbird. It is highly complex and needs: * Fast enough for up to 1 million entries * tree: hierarchical entries with expander * table: multiple columns, sortable, configurable * the combination of tree and table at the same time Without XUL <tree>, and without a suitable replacement that at the very minimum does all of that, (and of course more that is too extensive to discuss here), ***Thunderbird will simply be dead.*** This is why I asked bug 1183812 to be fixed. This will not be trivial, and won't be done in a weekend. I've already looked, and there's nothing out there that even comes close. This is a 6-12 months project with full time staff, given that so much of Thunderbird hangs off it. We need a suitable replacement that meets all requirements of Thunderbird, *before* XUL <tree> is removed. Otherwise the project is dead. Note that Thunderbird has 25 million users. I don't think it's worth it.
Paolo: I understand that this is just a placeholder and you don't have a timeline yet. But this needs a concerted effort to create a replacement, and is a large project on its own. If you want the Thunderbird project to handle this, we'd need 2 years of ahead warning time, and funds to create the replacement, and need to build a development team for it etc..
This is a tracking bug for the removal of the "tree" widget from mozilla-central. We can continue to discuss the Thunderbird picture in bug 1446341, where I just commented with some considerations and suggestions.
Moving to Core:XUL per https://bugzilla.mozilla.org/show_bug.cgi?id=1455336
Component: XP Toolkit/Widgets: XUL → XUL
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