[resize-observer] In shipping browsers, observe() doesn't call unobserve()

[iccir]

Per the current spec, observe(target, options) is suppose to call unobserve(target) if target is already in the observationTargets.

From my understanding, this should clear the lastReportedSizes array for the target. Hence, even if the target hasn't changed size, re-calling observe with the target should always fire an observation.

In practice, all major web browsers do not appear to implement this. Instead, to get the observation to fire, I must manually call unobserve(target) first and then call observe(target) again.

In this example, adding or removing a column should always trigger an observation. However, it only triggers an observation if I explicitly call unobserve() first.

[Loirooriol]

https://searchfox.org/firefox-main/rev/dcdbc805276cd5bf131cac2dbf53dda1538a32d5/dom/base/ResizeObserver.cpp#278-284 has this explanation:

      // Already observed this target and the observed box is the same, so
      // return.
      // Note: Based on the spec, we should unobserve it first. However,
      // calling Unobserve() when we observe the same box will remove original
      // ResizeObservation and then add a new one, this may cause an unexpected
      // result because ResizeObservation stores the mLastReportedSize which
      // should be kept to make sure IsActive() returns the correct result.

[iccir]

https://github.com/WebKit/WebKit/blob/6c7c731f37b5e5c3200d64d741291c0544b0c0cb/Source/WebCore/page/ResizeObserver.cpp#L79 has a similar note:

// The spec suggests unconditionally unobserving here, but that causes a test failure:
// https://github.com/web-platform-tests/wpt/issues/30708

Chromium has no such note, it bails out if the target is already observed. https://github.com/chromium/chromium/blob/b975af699670b7d458484a3a06a04bf7bed941c7/third_party/blink/renderer/core/resize_observer/resize_observer.cc#L76