Broken text entry in new screenshot manager

[YoshiRulz]

Your system information

• Steam client version (build number or date): 1726088054
• Distribution (e.g. Ubuntu): Ubuntu 22.04.5
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz
• GPU: AMD

Please describe your issue in as much detail as possible:

I expect text entry controls to behave similarly to every other program and UI framework: Once in a textbox, the arrow keys alone cannot take you out of it.

This probably isn't the place but I'd like to add that the film-strip-style layout was fine. The one before that didn't look great and sorely needed to get the UI scale, but in terms of functionality it was also fine. "If it ain't broke" and all that.

Steps for reproducing this issue:

1. Open screenshot manager with some screenshots
2. Share > Share on Steam...
3. Begin typing caption, then go back to the start with the Left Arrow key
4. The textbox loses focus

Same with Right Arrow at the end.
edit: To clarify, this is as soon as the cursor is at the start/end position. It's not that you move the cursor to the end and then an additional press will defocus the textbox.

see also #10667