[chiragjangiir]

Summary

TimedTextImsc11 dropped trailing forced line breaks on import: a cue written as C<br/><br/><br/> loaded as C. In TTML <br/> is a forced break with layout meaning (often paired with region extent / tts:displayAlign), so the cue that the file describes is not the cue the reader produced. This is the remaining half of #15289 — the extension-routing half was fixed in #15570.

Root cause

LoadSubtitle built each cue as:

var text = assStyle + ReadParagraph(node, headIndex).TrimEnd();

ReadParagraph() maps every <br/> to AppendLine(), so the trailing breaks were present in the string and then the blanket TrimEnd() removed them along with source formatting whitespace.

Reproduced on unmodified main with a reader-level test:

Assert.Equal() Failure: Strings differ
Expected: "C\n\n\n"
Actual:   "C"

The writer already serializes text faithfully (string.Join("<br/>", text.SplitToLines())), so the loss was reader-only: load → save at format level was not lossless.

Solution

Track the end of the last forced break while reading a paragraph, and trim only the text behind it:

• everything before that point already ends in a line break and is content, so it is left untouched — trailing <br/> breaks now survive;
• the tail is still source formatting and is trimmed exactly as before;
• a paragraph with no <br/> is trimmed exactly as before (raw.TrimEnd()), so behaviour for ordinary files is unchanged;
• nested <span> content is still read raw and trimmed only in the finished paragraph, so a space inside the last span still survives (Italic_Trailing_Space_Preserved).

No changes to timing, paragraph start/end values, or the writer.

Testing

Local: Apple Silicon macOS, .NET SDK 10.0.401.

Command	Result
dotnet test tests/libse/LibSETests.csproj --filter "FullyQualifiedName~TimedTextImsc11Test"	before fix: 2 failed (Trailing_Forced_Line_Breaks_Are_Preserved, LoadSaveLoad_Keeps_Trailing_Forced_Line_Breaks), 7 passed; after fix: 9/9 passed
dotnet build SubtitleEdit.sln --no-restore	exit 0, 0 errors, 0 warnings
dotnet test SubtitleEdit.sln --no-build (same as .github/workflows/tests.yml)	LibSETests 2588/2588 passed, SeConvTests 586 passed / 2 skipped, LibUiLogicTests 1215 passed, UITests 7380 passed / 6 failed

New regression tests:

1. Trailing_Forced_Line_Breaks_Are_Preserved — the reported case (A<br/><br/><br/>B and C<br/><br/><br/>).
2. Trailing_Source_Whitespace_Is_Still_Trimmed — guards the whitespace half of the old TrimEnd() (inline trailing space, indented paragraph).
3. LoadSaveLoad_Keeps_Trailing_Forced_Line_Breaks — load → ToText → load is now lossless.

git diff --check is clean; only src/libse/SubtitleFormats/TimedTextImsc11.cs and tests/libse/SubtitleFormats/TimedTextImsc11Test.cs are changed.

Limitations

• The 6 UITests failures are pre-existing on unmodified main on this machine: I re-ran exactly those tests with my changes stashed and the same 6 failed (SyntaxHighlightingTextPresenterCanaryTests ×3, RtlSplitRunHitTestTests, AssaTagRtlAvaloniaCanaryTests, TableViewScrollBarTroughPagingTests). They are headless-Avalonia rendering/style canaries unrelated to subtitle formats. CI runs on ubuntu-latest; the real CI result is the authority for that suite.
• GUI round trip still trims trailing line breaks — intentionally. SubtitleLineViewModel.ToParagraph() trims trailing whitespace on every save so a trailing Enter typed in the edit box can never reach a saved file (the cross-format invariant from Remove text for HI reports a trailing empty line as a change #13389). Distinguishing a loaded break from a typed one would need state the text box does not have, so I did not touch it. This PR makes the reader (and everything built on Subtitle — headless conversion, batch, plugins, the text shown in the editor) faithful; saving an untouched file from the GUI still applies the existing trimming rule.
• Tested on macOS only (no Windows machine available); no GUI/manual verification was performed — coverage is at the SubtitleFormat API level, which is the project's supported way to test readers.