This commits sets up the Table of Contents to bold the active section
when the section is scrolled.
Unfortunately, because our content does not have actual sections but
instead has a flat list of headings and paragraphs, we can't use
IntersectionObserver in the conventional way as it is optimized to find
intersections of elements that are *within* the viewport and the
callback will not reliably fire during certain scenarios (e.g. with fast
scrolling or when the headings are not currently within the viewport).
Furthermore, iterating through a list of elements and calling
`getBoundingClientRect()` can be expensive and can also cause
significant forced synchronous layouts that block the main thread.
The best compromise in terms of performance and function that I've found
is to use a combination of a throttled scroll event listener and
IntersectionObserver's ability to asyncronously find the
boundingClientRect of all elements off the main thread when `.observe`
is called which is the approach this patch takes. Although this is an
unorthodox way to use IntersectionObserver, performance profiles
recorded while holding the "down" arrow and scrolling for 10 seconds
with a 6x CPU throttle are comparable between master and this patch:
master: https://phabricator.wikimedia.org/F34930737
this patch: https://phabricator.wikimedia.org/F34930738
Bug: T297614
Change-Id: I4077d86a1786cc1f4a7d85b20b7cf402960940e7
Move A/B test code to AB.js
Consolidate the show/hide code spread across scrollObserver
and stickyHeader by adding a show and hide function.
This is needed to fix T296680
Change-Id: Ia2e0c50278df0dfc1600610f281be20f4cc755c2
- Permits logging for scroll events without sticky header.
- Update function name to be more precise.
Bug: T292586
Change-Id: I441b4bf81bc4a36a03f0f1c215d86b01dce2911d
- Pull IntersectionObserver into new file to share observer with different callbacks:
- Wrap show/hide functionality of sticky header in conditionals based on user test group or by default.
- Fire hooks for scroll event tracking in WME.
- Add new js for A/B test functions and variables:
- Fire hook to send data for A/B test initialization.
- Update main js to include scrollObserver, A/B test init functionality.
- Add A/B test config.
- Update ResourceLoader package dependencies for sticky header.
- Though not a strict dependency, see I42e3e7c2084c1e88363d5d1662630ed23a28c4d2 in WME repo which uses these hooks to log scroll events.
- This patch includes changes from I56f40e706f8706fde1c0891a0561dd32c5e02bfc which were consolidated here for simplicity and ease of review - related to T292587 which calls for logging an init event for bucketing of users during A/B testing.
Bug: T292586
Change-Id: If6446e1e84cea3649905808c4f0e9f6862255fa3
stickyHeader.js, a file in the "skins.vector.es6" module, clones the
user menu. Because of this, it must initialize before dropdownMenu.js, a
file in the "skins.vector.js" module, in order for dropdownMenu.js to
bind the correct checkboxHack event listeners to the user menu in the
sticky header.
Therefore, change the es6 module to export its main method. The
skins.vector.js module can then use mw.loader.using to ensure the
skins.vector.es6 module initialization happens first in browsers that
support es6. Browsers that don't support es6 will continue to initialize
the skins.vector.js module.
Bug: T291096
Change-Id: I1bb6f2da9703ed2679eacfdb42b9818efe614ab9
- Can now use const/let
- No need for feature detection for things like fetch and closest
as we can assume they exist if ES6 support is available
Change-Id: I85b01add13fd74e1514119498815403e42a09af0
This will allow us to write ES6 code for the new features which
is limited to those browsers.
For browsers that do not support ES6, the code will not execute
because of the "es6" flag. Doing this will help us avoid issues
like T293402
Change-Id: Iffb7098cb22395e33b87352fb4f08516f6f25e6f
