Did a quick `rr` on this - Seems that when you move the bottom edge of the window around, in [the observer loop](https://searchfox.org/mozilla-central/source/dom/base/ResizeObserverController.cpp#120-138): # GatherAllActiveObservations called with shallowestTargetDepth = 0 (Line 120) # Changes broadcasted, target depth updated, last reported size updated (Line 124) # Flush layout, window size changes again (Line 132, presumably) # GatherAllActiveObservations called with shallowestTargetDepth now set to the canvas depth - size changed again, but depth of canvas is not deeper than shallowestTargetDepth, the change is skipped (As per the standard) # Eventually we exit the loop, figure out that there are skipped observations, then send out the error (As per the standard) Not sure why this only happens 2 iframes deep.
Bug 1685038 Comment 7 Edit History
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Did a quick `rr` on this - Seems that when you move the bottom edge of the window around, in [the observer loop](https://searchfox.org/mozilla-central/source/dom/base/ResizeObserverController.cpp#120-138): * GatherAllActiveObservations called with shallowestTargetDepth = 0 (Line 120) * Changes broadcasted, target depth updated, last reported size updated (Line 124) * Flush layout, window size changes again (Line 132, presumably) * GatherAllActiveObservations called with shallowestTargetDepth now set to the canvas depth - size changed again, but depth of canvas is not deeper than shallowestTargetDepth, the change is skipped (As per the standard) * Eventually we exit the loop, figure out that there are skipped observations, then send out the error (As per the standard) Not sure why this only happens 2 iframes deep.