Fix: Chrome + iPad not creating rootElement early enough for custom frontends

[lukehb]

Relevant components:

• Frontend UI library

Problem statement:

The _rootElement in the Pixel Streaming UI library was not created explicitly, but rather created lazily by a getter called rootElement. There were some scenarios where you could trigger internal code paths that used the _rootElement before any call had been made via the getter to initialize it. This would result in a JS error for those calling functions.

Solution

This PR explicitly initializes the _rootElement in the constructor of the Pixel Streaming Application - this way the calling order is no longer important and it is always safe to access the _rootElement.

Test Plan and Compatibility

I have tested this locally using my Chrome browser, but did not have an iPad nearby to confirm. I will get someone to test this downstream once it is landed to confirm the fix is valid.

[github-actions]

💔 Some backports could not be created

Status	Branch	Result
❌	UE5.4	Backport failed because of merge conflicts You might need to backport the following PRs to UE5.4: - Better Safari detection and removing overflow on iPad
❌	UE5.3	Backport failed because of merge conflicts You might need to backport the following PRs to UE5.3: - Better Safari detection and removing overflow on iPad
✅	UE5.5

Note: Successful backport PRs will be merged automatically after passing CI.

Manual backport

To create the backport manually run:

backport --pr 410

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details

[mcottontensor]

💚 All backports created successfully

Status	Branch	Result
✅	UE5.3
✅	UE5.4

Note: Successful backport PRs will be merged automatically after passing CI.

Questions ?

Please refer to the Backport tool documentation