text: parse prefix extensions incrementally in TextViewState::set_text (#3291)

[Yi-111-a]

Problem

TextView::markdown(id, text) hands its text to the keyed state every frame, through set_element_text → set_text. When the new text extends the current one, set_text noticed text.starts_with(self.text), but only used that to note stream fade before calling increment_update(text, false, cx), which performed a full replacement reparse:

• Up to MAX_SYNC_FULL_REPLACE_BYTES (4KB), it parsed synchronously on the UI thread and sent a baseline ack to the background parser;
• Above 4KB, it parsed in the background and invalidated measured heights for all blocks upon landing;
• It bumped selection_revision, resetting active selections on every streaming token growth;
• Over a multi-token stream (e.g. streaming LLM outputs in ai-chat), every update cost $O(n)$, totaling $O(n^2)$ time over the entire stream.

Solution

In TextViewState::set_text:

1. When !self.text.is_empty() && text.starts_with(self.text.as_str()), take the incremental append path matching push_str:
   • Compute delta = &text[self.text.len()..];
   • Note stream fade extension at self.text.len();
   • Append delta to self.text;
   • Dispatch self.increment_update(delta, true, cx) so only the trailing block is reparsed without resetting selection or invalidating earlier measured block heights.
2. Keep full replacement for initial updates (self.text.is_empty()), text truncation, or non-prefix replacements.

Validation

• Added set_text_prefix_extension_updates_incrementally_like_push_str testing that prefix extensions via set_text retain selection_revision and full_update_revision (unlike replacements).
• Added set_text_streaming_matches_push_str_parsed_content verifying parity between streaming chunks through set_text vs push_str across multi-paragraph, inline code, and list structures.
• Ran cargo test -p gpui-base text::state (81 tests passed).
• Ran cargo clippy -p gpui-base (0 errors).

Closes #3291

Fixes #3291

[madcodelife]

Thanks for the fix!

I pushed a follow-up commit: set_text now appends incrementally only for Markdown. HTML, and any update after a failed parse, still reparses in full, because appending gives wrong results in those cases. I also added tests that check streaming against a full parse.