This implements the first part - supporting empty tiles (holes) in surfaces. I'm working on the second part (per-tile clip rects to reduce pixel count further) as a follow up.

Since applying the patch, I saw a white rectangle during scrolling a page or Tab switching. I wonder if the problem is related to the patch. I do not know a specific STR to reproduce it reliably yet.

In D59059#1799747, @sotaro wrote:

Since applying the patch, I saw a white rectangle during scrolling a page or Tab switching. I wonder if the problem is related to the patch. I do not know a specific STR to reproduce it reliably yet.

OK, I won't merge this for now.

https://www.yahoo.com/ was easier to reproduce the problem for me. I confirmed it on 2 Win10 PCs. Without the patch, I did not see the problem.
STR

• [0] Enable wr os compositor with the patch.
• [1] Enable bookmark toolbar
• [2] Bookmark https://www.yahoo.com/ to the toolbar
• [3] click the yahoo link on the toolbar. And load https://www.yahoo.com/
• [4] Scroll the page

When the problem did not happen at [4], the repeat [3] and [4].

In D59059#1799790, @sotaro wrote:

https://www.yahoo.com/ was easier to reproduce the problem for me. I confirmed it on 2 Win10 PCs. Without the patch, I did not see the problem.
STR

• [0] Enable wr os compositor with the patch.
• [1] Enable bookmark toolbar
• [2] Bookmark https://www.yahoo.com/ to the toolbar
• [3] click the yahoo link on the toolbar. And load https://www.yahoo.com/
• [4] Scroll the page

When the problem did not happen at [4], the repeat [3] and [4].

I can repro this locally - I will investigate today / tomorrow. Thanks for the repro!

Updated with a fix for a typo in the UpdateAllocatedRect method. Seems to fix the problem for me.

Updated patch addressed the problem also for me!