Desktop renderer freezes on large file diffs (synchronous diff parsing on UI thread) #34437

Problem

OpenCode Desktop freezes on Windows when working with large file diffs — for example, C++ projects like llama.cpp or sessions with many modified files. The renderer becomes unresponsive for seconds at a time.

Root Cause

execEditLength runs synchronously on the UI render thread during diff computation. The call chain:

SessionReview → normalize → resolveFileDiff → parseDiffFromFile → structuredPatch → diffLines → execEditLength

Session Review eagerly parses every file while building reactive component state, before expansion or virtualization. A sufficiently large diff can block the renderer for hundreds of milliseconds while messages continue streaming.

Impact

• Windows users experience multi-second freezes when opening sessions with large diffs
• Committed 200ms responsiveness budget fails (max frame gap: 233ms)
• Makes OpenCode Desktop nearly unusable for C++ or other large-file projects

Fix

PR #31309 by @Hona moves diff preparation off the render thread into a Web Worker, reducing max frame gap from 233ms to 67ms (71% reduction). A clean rebase is available at #34415