Fold unchanged regions in the unified diff - #2791
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This is a draft so that @tobiasmelcher can join in if he wants. Current state: Folding works but I have no way to unfold. 
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Add a foldUnchanged(contextLines) option to UnifiedDiff that collapses the unchanged gaps between changes, keeping a few context lines around each change, similar to the unified view on GitHub. It reuses the editor's projection (folding) model, so it is a no-op when folding is disabled, and its folds are tagged so they can be removed without touching the editor's own folds. Each collapsed region shows a clickable "Expand n unchanged lines" code mining that expands it in place. Enabled with three context lines on both unified diff paths.
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This adds folding of unchanged regions to the experimental unified diff overlay, similar to the collapsed unchanged sections in GitHub's diff view.
A new
UnifiedDiff.Builder#foldUnchanged(contextLines)option collapses the unchanged gaps between the displayed changes, keeping a few context lines visible around each change; it is enabled with three context lines where the unified diff is opened.Each collapsed region shows a clickable "Expand n unchanged lines" code mining that expands it in place; re-collapsing via the folding ruler brings the expander back.
The implementation reuses the editor's projection (folding) model and the folds are tagged with a marker annotation so they can be removed on overlay teardown (Hide All Diffs, switching to the 2-way editor, dismissing the last diff) without touching the editor's own structural folds.
This makes reviewing large files with few changes considerably more compact.
Still a draft for one reason: reusing the projection model means this only takes effect in editors that install projection support, such as the Java editor or the Generic Editor.
AbstractDecoratedTextEditorandTextEditordo not, so for a plain text file opened with the default text editor the option is silently a no-op. That needs a decision before this lands: accept it as a limitation, or fall back to something that does not depend on projection.Part of eclipse-platform/eclipse.platform.ui#3771