Internal `rangechange` event firing too frequently when scrolling near top of list

[graynorton]

Found in: 0.7.0

Expected Behavior:

The internal rangechange event should fire with similar frequency regardless of the current scroll position.

Actual behavior:

rangechange fires much more frequently when scrolled to the top of the list

Repro steps:

1. Open https://tinyurl.com/3aapymvz

2. In Chrome Dev Tools, locate the Virtualizer.js file in the Sources tab and open it:

3. Within that file, find the line containing case 'rangechange' and add a logpoint on the next line (just log anything, doesn't matter what):

4. In Dev Tools, switch to the Console tab.

5. In the playground's Result pane, scroll down slowly. You'll see lots of logging to the console, more or less with every pixel you scroll.

6. Scroll more quickly down the view, until 'Item 1' or later becomes visible.

7. Clear the console, then resume scrolling slowly down.

=> You'll see that logging happens much less frequently.

The behavior near the top of the list is incorrect; the frequency of rangechange events is too high.

Notes:

This seems to be due to a bug introduced when adding support for margin-collapsing, in this commit: ca42077

Specifically, we aren't treating margins consistently when calculating the internal physicalMin and physicalMax state variables that are used to trigger layout updates.

This bug has already been fixed; just writing it up for review / posterity.

[AndrewJakubowicz]

This is an incredibly well written issue and repro. Great easy to follow steps. Thank you!
TIL logpoint.