fix - reduce streaming render cost with incremental layout and event batching

[xinyi-gong]

fix #259

Root cause

Every streaming chunk (text fragment, thinking update, tool call delta) triggered a full
layout(true, true) + cmpContent.computeSize(SWT.DEFAULT, SWT.DEFAULT) pass that re-measured
every historical turn.

Fixes

1. Event-drain batching (drainPendingEvents)

Streaming events are now queued into a ConcurrentLinkedQueue and drained in a single scheduled
UI-thread callback instead of dispatching one layout pass per event. A re-entrancy guard prevents
cascading drains from controlResized.

2. Incremental / O(1) layout

During streaming only the trailing turns (latest user turn + active Copilot turn) are
re-measured. Historical sealed turns keep their cached sizes (computeSize(w, DEFAULT, false)).
This replaces the previous layout(true, true) call that invalidated the entire composite.

3. Constrained-width size measurement (flicker fix)

computeSize is now called with the actual clientArea.width instead of SWT.DEFAULT.
Previously the unconstrained width produced a phantom over-estimated height, causing the scroll
position to oscillate during streaming and producing visible flicker.

4. Drop per-chunk enclosing-scroller refresh in ThinkingBlock streaming

The thinking streaming path no longer calls refreshEnclosingScroller() on every chunk. Long
thinking content previously forced a full enclosing ChatContentViewer re-layout per chunk
(hundreds of full O(n) passes per turn). The enclosing scroller is now refreshed by the batched
drainPendingEvents → doRefreshScrollerLayout path along with all other streaming events; the
local updateScrollerDuringStreaming() only manages the thinking block's own inner scroller. The
expand/collapse path still refreshes the enclosing scroller directly.

5. Incremental re-measure on turn-end only

The drain batch triggers a incremental re-measure only when it contains a turn-end event, ensuring
minHeight is accurate before scrollToBottom().

Performance results

Benchmark: deterministic repeated-Ask prompt, 5 turns (n = widget count, increments by 2 per turn).
Compared against baseline run on unfixed HEAD.

turn	refresh avg (ms) base → fix	refresh total (ms) base → fix	render % of wall-clock base → fix
n2	20.04 → 5.49	13 650 → 3 168	33% → 10%
n4	34.38 → 5.34	23 960 → 2 628	55% → 9%
n6	46.79 → 9.01	38 414 → 5 787	68% → 15%
n8	52.19 → 6.58	42 011 → 4 329	70% → 11%
n10	62.47 → 9.56	49 910 → 6 464	77% → 17%

~7.7× reduction in layout time at turn 5. The per-pass average is now essentially flat
(5.5–9.6 ms) across n2–n10 — effectively O(1) in turn count. Per turn only 2–4 full-measure
passes run; the rest take the O(1) incremental path. Render is no longer the dominant cost.

Perf Doc: perf-baseline-259.md

Files changed

• ChatContentViewer.java — drain queue, incremental layout, re-entrancy guard, flicker fix,
  full-measure-on-turn-end, auto-scroll logic
• ThinkingBlock.java — parametrised refreshEnclosingScroller(boolean incremental)
• BaseTurnWidget.java — minor cleanup
• ChatMarkupViewer.java — minor cleanup

[Copilot]

Pull request overview

This PR addresses performance regressions in the chat UI’s streaming render path by batching incoming progress events and reducing layout work during streaming, aiming to restore responsive agent interactions (issue #259).

Changes:

• Batch LSP streaming/progress events via a pending queue drained on the UI thread to avoid per-chunk full relayouts.
• Introduce incremental (trailing-turn-only) layout/measurement and use constrained-width sizing to reduce flicker and layout cost.
• Remove per-chunk enclosing-scroller refresh from ThinkingBlock streaming updates, relying on the batched refresh path instead.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ChatContentViewer.java	Adds queued/batched event draining and incremental scroller layout to reduce streaming render cost and flicker.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ThinkingBlock.java	Avoids refreshing the enclosing scroller on every streaming chunk.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/BaseTurnWidget.java	Updates refresh scheduling call site to the renamed refresh scheduler.