fix: chunk large JCEF messages to prevent JetBrains sidebar freezes

[RomneyDa]

Summary

• When sendToWebview is called with a large payload (e.g. loading a long conversation session), the entire JSON is embedded in a single executeJavaScript("window.postMessage(<huge JSON>, '*')") call. JCEF's JS source parser/compiler cannot handle multi-megabyte source strings and freezes the IDE.
• This adds a chunked path for messages over 1MB: the JSON is base64-encoded, split into 2MB chunks sent as separate executeJavaScript calls that each push to an array buffer, then one final small call joins, decodes, and dispatches the complete message via window.postMessage.
• Messages under 1MB (the vast majority) take the existing fast path, unchanged.

How the chunking works

JCEF never sees partial JSON. Each executeJavaScript call just pushes a base64 string fragment to an array (buffer.push("abc123...")). No JSON parsing happens until the very last call, which runs JSON.parse(atob(buffer.join(""))) on the reassembled complete string. The freeze was caused by JCEF having to compile a massive JS source string, not by V8 processing the data — so keeping each source string small (~2MB) avoids the freeze entirely.

Array push+join is used instead of string concatenation to keep reassembly O(n) instead of O(n²). The chunk generation logic is extracted into a testable buildChunkScripts() function with unit tests.

Related issues

Fixes the large-session-file variant of the JCEF freeze reported across multiple issues:

• [JetBrains] Sidebar panel frequently freezes across all OS' #8085 — [JetBrains] Sidebar panel frequently freezes across all OS' (user confirmed freeze correlates with session file size approaching 100MB, and that deleting/shortening session files resolves it)
• Continue plugin freezes #8587 — Continue plugin freezes (user confirmed freezes when session file exceeds ~93,000 KB)
• Continue is not working (NPE since IntelliJ update to 2025.2 #7285 — Continue is not working (NPE since IntelliJ update to 2025.2) (related JCEF overload)
• JetBrains Plugins use #9159 — JetBrains Plugin unresponsive (JCEF browser not initialized after overload)
• BUG in Pycharm #10069 — BUG in Pycharm (same JCEF crash pattern)

Test plan

• Open a JetBrains IDE with Continue installed
• Have a normal short conversation — verify messages display correctly (fast path)
• Load a session with a very long conversation history (large session JSON file) — verify it loads without freezing the IDE
• Verify the sidebar remains responsive after loading a large session
• Run ./gradlew test --tests "*.ContinueBrowserChunkTest" — all 7 unit tests pass

[continue]

Docs Review: No documentation updates needed.

This PR is an internal bug fix to the JetBrains extension's JCEF message handling. The chunking mechanism is:

• Completely transparent to users (no new APIs or config options)
• An implementation detail that doesn't affect how developers use Continue
• A performance/stability fix that users benefit from automatically

The existing docs accurately describe using Continue in JetBrains — they don't need to explain internal message transport mechanisms.

[cubic-dev-ai]

1 issue found across 1 file

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="extensions/intellij/src/main/kotlin/com/github/continuedev/continueintellijextension/browser/ContinueBrowser.kt">

<violation number="1" location="extensions/intellij/src/main/kotlin/com/github/continuedev/continueintellijextension/browser/ContinueBrowser.kt:92">
P2: Escape or serialize messageId before embedding it into JavaScript string literals. As written, a messageId containing quotes/backslashes will break the generated JS (or allow injection) in the chunked path.</violation>
</file>

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