Since v0.10.0 chat messages are virtualized with content-visibility: auto to skip rendering of off-screen messages. Safari's implementation of content-visibility has known paint bugs (WebKit bugs 277573, 281570 and 283846) that can leave the contents of a message unpainted even when it is on screen. On iOS this makes assistant responses render as empty, both in Safari and as a PWA, while the same chats render fine in Chromium and Firefox. This matches the regression window reported in #26712, which appeared when upgrading from 0.9.6 to 0.10.2.
Detect Safari (including all iOS browsers, which use WebKit) with the same user agent check already used in MessageInput and ShareChatModal, and skip the virtualization class there. Safari falls back to rendering all messages like before v0.10.0, while other engines keep the optimization. Verified with a spoofed Safari user agent that messages render without the virtualization class and with content-visibility resolving to visible, while Chromium keeps content-visibility: auto.
Fixes#26712
Assistant responses are now stored as structured output items on message.output, with message.content left empty. The action payload built in chatActionHandler still sent message.content only, so action functions received assistant messages with an empty content property. Derive the content from the structured output via getOutputText, falling back to message.content, matching how the rest of Chat.svelte resolves assistant text.
Fixes#26672
