Make SubtitleLineViewModel.StartTime a plain setter; add StartTimeKeepDuration

[niksedk]

Background

SubtitleLineViewModel.StartTime's setter had a hidden side effect: assigning it also moved EndTime to preserve the duration ("drag the whole line"). This made the most natural-looking statement — line.StartTime = x — the dangerous one:

• Programmatic code almost always wants the end to stay fixed, so the codebase had ~45 SetStartTimeOnly(...) calls to opt out of the drag.
• Forgetting SetStartTimeOnly and writing StartTime = x silently shifted the end too, corrupting timing.

This is the design follow-up to #11661 (which routed Adjust through SetTimes). It inverts the default so the safe behavior is the obvious one.

Change

• StartTime setter is now plain: it moves the start, keeps the end fixed, and recomputes the duration — identical to the existing SetStartTimeOnly, which is kept as an alias so the ~45 existing callers are unchanged.
• New StartTimeKeepDuration property provides the old drag-the-line (preserve-duration) behavior, explicitly.
• Grid binding (InitListViewAndEditBox): the start-time editor binds to StartTimeKeepDuration when no separate end-time editor is shown (preserving that UX), and to StartTimeOnly when one is.

Caller audit

I audited every direct SubtitleLineViewModel.StartTime = … assignment in src/ui (excluding Paragraph and other types). Only five relied on the drag and were converted to StartTimeKeepDuration:

• AudioVisualizer.cs — waveform Alt-drag (move whole line)
• MainViewModel.cs — insert subtitle at video position
• MainViewModel.cs — transcribe split into new lines
• MainViewModel.cs — ripple-delete trailing-line shift
• PointSyncer.cs — single-point sync

All other direct assignments either set EndTime immediately after or wanted keep-end semantics (one, snap-start-to-shot-change, is actually fixed by the new plain setter).

Bonus fix

Converting single-point sync (AdjustViaShowEarlierLater) to StartTimeKeepDuration also fixes a real bug: it previously shifted only the start time of every line, leaving end times in place, which shrank/inverted durations. It now shifts the whole line by the offset.

Tests

• SubtitleLineViewModelTimeTests — StartTime (plain, keep-end), SetStartTimeOnly (keep-end), and StartTimeKeepDuration (positive/negative shift, duration preserved).
• PointSyncerTests — single-point sync shifts both start and end, preserving durations.

Full UI suite: 354/354 passing.

🤖 Generated with Claude Code

[niksedk]

Self-review follow-up

Ran a multi-angle review of this branch. It caught one significant miss in the original caller audit, now fixed in bd30cfe:

The audit grepped for .StartTime = and so never matched the compound-assignment form StartTime += …. Six whole-line-shift sites use exactly that and relied on the old end-follows-start behavior — after the inversion they would have moved only the start and corrupted durations:

• Adjust(...) — all three modes (all / selected / selected-and-forward), i.e. the "Show earlier/later" / "Adjust all times" sync tool
• WaveformSetStartAndOffsetTheRest / WaveformSetEndAndOffsetTheRest
• AudioVisualizerSetStartAndOffsetTheRest

All converted to StartTimeKeepDuration.

Also simplified StartTimeKeepDuration to delegate to SetTimes(value, value + (EndTime - StartTime)) — applies start/end atomically (no transient start > end) and computes the span from live times instead of the Duration field.

Other review findings reviewed and not changed (not regressions from this PR):

• Plain StartTime can now produce a negative duration if start is set past end — but this was already true via the pre-existing StartTimeOnly binding (end-editor mode) and SetStartTimeOnly; not introduced here.
• _skipUpdate early-return dropping a two-way write-back — refuted: the suppression windows are synchronous on the UI thread, so user input can't interleave; StartTimeOnly already carried the identical guard.
• Gap/neighbor brush staleness after a keep-duration drag — same as the previous drag setter (duration unchanged ⇒ no brush re-notify); gap coloring is refreshed by the slow timer.

Full UI suite: 354/354 green.