[TextBoxView] Crash during large selection while background layout is incomplete

[etvorun]

[TextBoxView] Crash during large selection while background layout is incomplete

Summary

TextBoxView.GetLineIndexFromOffset can fail during large-text selection when layout is still in time-sliced background progress, causing a crash/fail-fast path instead of graceful handling.

Observed behavior

• Selecting text in very large TextBox content (for example, very long lines / high offsets) can trigger a consistency/assert failure.
• The failure happens when selection/highlight processing requests an offset beyond the currently measured line range.

Repro notes (public)

1. Create/open a WPF app view with a TextBox containing a very large amount of text (for example, one very long line or total offsets in 100K+ range).
2. Trigger selection operations while layout is still being processed incrementally (IME/search/focus interactions can increase likelihood).
3. Observe crash/fail-fast behavior from TextBoxView.GetLineIndexFromOffset path.

Impact

• Affects reliability of text editing/selection in large-document scenarios.
• More likely in interactive flows where layout and selection/focus updates interleave.

Expected behavior

Selection/highlight should remain stable and never crash, even when background layout has not measured all lines yet.

Actual behavior

The lookup assumes the requested offset is always inside measured _lineMetrics, and can fail when it is temporarily outside that range.

Suspected root cause

TextBoxView performs time-sliced layout, so _lineMetrics may be only partially populated. During that window, selection/highlight can request an offset greater than the last measured line EndOffset, and the search path fails.

Proposed fix direction

• Add defensive clamping in GetLineIndexFromOffset for offsets beyond currently measured range.
• Return the last measured line index when background layout is pending.
• Keep correctness checks for unexpected conditions while avoiding crash in expected incremental-layout windows.
• Remove redundant preconditions that incorrectly assume full containment during transitional layout states.