Created attachment 278424 [details] A small HTML file reproducing the bug. In RTL, a set of inline elements within a horizontally-scrollable container will flow from right to left, and the container starts scrolled all the way to the right. With default scrolling, adding a 3d transform to the items in the scrollable container causes no problem. However, on iOS safari, adding -webkit-overflow-scroll: touch to the scrollable container causes any elements of the scroller with a 3d transform to be rendered in the wrong position. For this bug to occur, there must be enough elements in the scrollable container that the container can actually scroll. Viewing the buggy page with the web inspector shows that the browser thinks the elements are in the correct place (and their tap targets still exist in the correct place as well), but the elements are drawn in the wrong position. I've attached a small HTML file which reproduces the issue.
<rdar://problem/26204794>
I cannot reproduce with the attached testcase on an iPad mini with 9.3.1 (making #scroller narrow enough to scroll). Note that the test case has no 3d-transformed items.
Created attachment 278541 [details] Fixed testcase
I can with the revised test case
Created attachment 278553 [details] Patch
Comment on attachment 278553 [details] Patch Attachment 278553 [details] did not pass mac-ews (mac): Output: http://webkit-queues.webkit.org/results/1300958 New failing tests: compositing/rtl/rtl-with-transformed-descendants.html
Created attachment 278558 [details] Archive of layout-test-results from ews103 for mac-yosemite The attached test failures were seen while running run-webkit-tests on the mac-ews. Bot: ews103 Port: mac-yosemite Platform: Mac OS X 10.10.5
Comment on attachment 278553 [details] Patch Attachment 278553 [details] did not pass mac-wk2-ews (mac-wk2): Output: http://webkit-queues.webkit.org/results/1300956 New failing tests: compositing/rtl/rtl-with-transformed-descendants.html
Created attachment 278559 [details] Archive of layout-test-results from ews104 for mac-yosemite-wk2 The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews. Bot: ews104 Port: mac-yosemite-wk2 Platform: Mac OS X 10.10.5
Comment on attachment 278553 [details] Patch Attachment 278553 [details] did not pass mac-debug-ews (mac): Output: http://webkit-queues.webkit.org/results/1300992 New failing tests: compositing/rtl/rtl-with-transformed-descendants.html
Created attachment 278562 [details] Archive of layout-test-results from ews115 for mac-yosemite The attached test failures were seen while running run-webkit-tests on the mac-debug-ews. Bot: ews115 Port: mac-yosemite Platform: Mac OS X 10.10.5
https://trac.webkit.org/r200665