Matches UiButton.cs:896-906 / UiElement.cs:683-720's own clip
discipline: every per-cell PushClip in DrawColumns now has its matching
PopClip in a finally, so a cell draw that throws (e.g. a plugin's icon
resolver misbehaving) doesn't leak that PushClip onto the context's
clip stack. Without this, the leaked entry combines badly with
UiElement.DrawSelfAndChildren's own outer clip pop on the way out
(it pops the wrong stack entry), permanently corrupting the shared
UiRenderContext's clip state for every draw that follows in the frame.
Added UiRenderContext.ClipStackDepth (internal, InternalsVisibleTo
AcDream.App.Tests) purely to make this provable from a test — the
number of PushClip calls not yet matched by PopClip.
New test: a column whose icon resolver throws mid-draw still leaves
the clip stack at its pre-draw depth after the exception propagates —
shown to fail first (leaked to depth 1 instead of 0, reproducing
exactly the "outer pop consumes the wrong stack entry" mechanism
described above) before the try/finally was added.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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