Improve diff highlight contrast in Eclipse dark theme

team/bundles/org.eclipse.compare/HANDOVER.md

@@ -0,0 +1,93 @@
+# Compare Bundle — Handover Notes
+
+## Dark Theme Diff Colors
+
+### Problem
+
+The diff highlight colors in the Compare editor (both 2-way compare and unified diff) have
+poor contrast in Eclipse Dark Theme. The issue was reported in
+[eclipse-platform#2993](https://github.com/eclipse-platform/eclipse.platform/issues/2993).
+
+### Color Architecture
+
+Both the **2-way compare** (`TextMergeViewer`) and the **unified diff code mining**
+(`UnifiedDiffCodeMiningProvider`) derive their diff background colors from two base colors
+registered in the JFace color registry:
+
+| Registry key    | Default value     | Configured via                          |
+|-----------------|-------------------|-----------------------------------------|
+| `ADDITION_COLOR` | `COLOR_DARK_GREEN` | Eclipse Preferences → Colors and Fonts |
+| `DELETION_COLOR` | `COLOR_RED`        | Eclipse Preferences → Colors and Fonts |
+
+These base colors are intentionally **not used directly** as background colors. Instead,
+`TextMergeViewer.ColorPalette` derives three tints by interpolating the base color toward
+the editor background:
+
+```
+normal   = interpolate(base, background, scale_normal)   // stroke/border
+fill     = interpolate(base, background, scale_fill)     // line background
+textFill = interpolate(base, background, scale_textFill) // inline text highlight
+```
+
+`UnifiedDiffCodeMiningProvider` uses the same interpolation for the code mining (overlay)
+path. The annotation painter path reads final colors directly from the preference store
+keys `inlineDiffColor` / `deletionInlineDiffColor` etc., which are overridden per-theme
+via `resources/css/dark.css`.
+
+### The Dark Theme Problem
+
+With a light theme background (~`255,255,255`) the interpolation at scale 0.9 produces a
+soft pastel tint — clearly visible. With a dark theme background (~`30,31,34`) the same
+scale produces a color nearly indistinguishable from the background. For example:
+
+- `DELETION_COLOR` = `COLOR_RED` = `(255,0,0)`, dark bg = `(30,31,34)`
+- fill at scale 0.9 → `(52,27,30)` — very dark, barely visible
+
+It is **not possible** to reach a well-contrasted dark theme color by only changing the
+base color (`ADDITION_COLOR` / `DELETION_COLOR`), because the 0.9 scale washes out any
+base color. Working backwards, even pure `(255,0,0)` only yields `(52,27,30)` as the fill.
+
+### Solution
+
+We preserve the existing user-configurable color architecture (users set `ADDITION_COLOR`
+/ `DELETION_COLOR` in Preferences and the three tints are derived from those). To improve
+dark theme contrast, the **scale factors are reduced when the dark theme is active**:
+
+| Theme | `normal` scale | `fill` scale | `detail` scale |
+|-------|---------------|-------------|----------------|
+| Light | 0.6           | 0.9         | 0.8            |
+| Dark  | 0.3           | 0.79        | 0.5            |
+
+The scales are centralised in three static methods `normalScale(boolean dark)`,
+`fillScale(boolean dark)` and `detailScale(boolean dark)` on
+`UnifiedDiffCodeMiningProvider`, which all three painting paths call.
+
+Dark theme detection uses `INFORMATION_BACKGROUND_COLOR` from the theme's color registry
+as a proxy (sum of RGB channels < 3×128 = dark).
+
+Note: `ADDITION_COLOR` and `DELETION_COLOR` are **JFace color registry keys**, not
+preference store keys. They cannot be overridden via the `IEclipsePreferences` CSS
+mechanism in `e4-dark_preferencestyle.css` — that mechanism only affects preference store
+entries. The default values `COLOR_DARK_GREEN=(0,128,0)` and `COLOR_RED=(255,0,0)` are
+therefore always the base colors fed into the interpolation.
+
+With the reduced dark scale factors these produce:
+
+| Color | Target (Option C) | Produced |
+|-------|-------------------|---------|
+| Addition fill   | `(13,59,13)`  | `(23,51,26)`  |
+| Addition detail | `(20,82,20)`  | `(15,79,17)`  |
+| Deletion fill   | `(59,13,13)`  | `(77,24,26)`  |
+| Deletion detail | `(82,20,20)`  | `(142,15,17)` |
+
+The annotation painter path (`dark.css`) is updated to match the same computed values so
+that both rendering paths produce consistent colors.
+
+### Files Changed
+
+| File | Change |
+|------|--------|
+| `compare/org/eclipse/compare/unifieddiff/internal/UnifiedDiffManager.java` | `UnifiedDiffPaintListener` uses dark-aware scale factor |
+| `compare/org/eclipse/compare/contentmergeviewer/TextMergeViewer.java` | `ColorPalette` uses dark-aware scale factors |
+| `compare/org/eclipse/compare/unifieddiff/internal/UnifiedDiffCodeMiningProvider.java` | Code mining path uses dark-aware scale factors; `isDarkTheme()` helper added |
+| `resources/css/dark.css` | Annotation painter colors updated to match computed values |

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/contentmergeviewer/TextMergeViewer.java

@@ -364,9 +364,10 @@ public ColorPalette(RGB background, String registryKey, RGB defaultRGB) {
 			} else {
 				selected = defaultRGB;
 			}
-			normal = DiffColors.interpolate(selected, background, DiffColors.BORDER_SCALE);
-			fill = DiffColors.interpolate(selected, background, DiffColors.FILL_SCALE);
-			textFill = DiffColors.interpolate(selected, background, DiffColors.TEXT_FILL_SCALE);
+			boolean dark = DiffColors.isDarkTheme();
+			normal = DiffColors.interpolate(selected, background, DiffColors.borderScale(dark));
+			fill = DiffColors.interpolate(selected, background, DiffColors.fillScale(dark));
+			textFill = DiffColors.interpolate(selected, background, DiffColors.detailScale(dark));
 		}
 	}
 

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/internal/DiffColors.java

@@ -14,6 +14,7 @@
 package org.eclipse.compare.internal;
 
 import org.eclipse.swt.graphics.RGB;
+import org.eclipse.ui.PlatformUI;
 
 /**
  * The shared color palette for diff rendering. Both the traditional two-way
@@ -35,9 +36,33 @@ private DiffColors() {
 	/** The hunk fill band; the lightest of the three. */
 	public static final double FILL_SCALE = 0.9;
 
-	/** Border color of an addition/deletion hunk; darker than the fill band. */
+	private static final String INFORMATION_BACKGROUND_COLOR = "org.eclipse.ui.workbench.INFORMATION_BACKGROUND"; //$NON-NLS-1$
+
+	/** Returns {@code true} when the active workbench theme is dark. */
+	public static boolean isDarkTheme() {
+		var reg = PlatformUI.getWorkbench().getThemeManager().getCurrentTheme().getColorRegistry();
+		var color = reg.getRGB(INFORMATION_BACKGROUND_COLOR);
+		return color != null && color.red + color.green + color.blue < 3 * 128;
+	}
+
+	/** Scale for the border/stroke color; adapts to dark vs. light themes. */
+	public static double borderScale(boolean dark) {
+		return dark ? 0.3 : BORDER_SCALE;
+	}
+
+	/** Scale for the line-band fill; adapts to dark vs. light themes. */
+	public static double fillScale(boolean dark) {
+		return dark ? 0.79 : FILL_SCALE;
+	}
+
+	/** Scale for the word-level detail highlight; adapts to dark vs. light themes. */
+	public static double detailScale(boolean dark) {
+		return dark ? 0.5 : TEXT_FILL_SCALE;
+	}
+
+	/** Border color of an addition/deletion hunk; adapts to dark vs. light themes. */
 	public static RGB borderColor(RGB diffColor, RGB background) {
-		return interpolate(diffColor, background, BORDER_SCALE);
+		return interpolate(diffColor, background, borderScale(isDarkTheme()));
 	}
 
 	/**

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/unifieddiff/internal/UnifiedDiffCodeMiningProvider.java

@@ -157,8 +157,9 @@ public CompletableFuture<List<? extends ICodeMining>> provideCodeMinings(ITextVi
 				this.foldButtonColor.dispose();
 			}
 			// the word-level diff is tinted stronger so it stands out against the band
-			this.detailedDiffColor = new Color(interpolate(deletionColor, background, DiffColors.TEXT_FILL_SCALE));
-			this.deletionBackgroundColor = new Color(interpolate(deletionColor, background, DiffColors.FILL_SCALE));
+			boolean dark = DiffColors.isDarkTheme();
+			this.detailedDiffColor = new Color(interpolate(deletionColor, background, DiffColors.detailScale(dark)));
+			this.deletionBackgroundColor = new Color(interpolate(deletionColor, background, DiffColors.fillScale(dark)));
 			this.borderColor = new Color(UnifiedDiffText.borderColor(deletionColor, background));
 			this.foldSeparatorColor = new Color(separatorBackground(background));
 			this.foldButtonColor = new Color(buttonBackground(background));

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/unifieddiff/internal/UnifiedDiffManager.java

@@ -31,6 +31,7 @@
 import org.eclipse.compare.contentmergeviewer.ITokenComparator;
 import org.eclipse.compare.contentmergeviewer.TokenComparator;
 import org.eclipse.compare.internal.CompareMessages;
+import org.eclipse.compare.internal.DiffColors;
 import org.eclipse.compare.internal.DocLineComparator;
 import org.eclipse.compare.rangedifferencer.IRangeComparator;
 import org.eclipse.compare.rangedifferencer.RangeDifference;
@@ -1822,7 +1823,8 @@ public UnifiedDiffPaintListener(ITextViewer viewer, IAnnotationModel model, Unif
 				color = JFaceResources.getColorRegistry().getRGB("ADDITION_COLOR"); //$NON-NLS-1$
 			}
 			RGB background = UnifiedDiffCodeMiningProvider.getBackground();
-			RGB interpolated = UnifiedDiffCodeMiningProvider.interpolate(color, background, 0.9);
+			boolean dark = DiffColors.isDarkTheme();
+			RGB interpolated = DiffColors.interpolate(color, background, DiffColors.fillScale(dark));
 			this.additionBackgroundColor = new Color(interpolated);
 			this.borderColor = new Color(UnifiedDiffText.borderColor(color, background));
 		}

team/bundles/org.eclipse.compare/resources/css/dark.css

@@ -1,7 +1,7 @@
 IEclipsePreferences#org-eclipse-ui-editors:org-eclipse-compare {
 	preferences:
-		'inlineDiffColor=27,40,30'
-		'detailedInlineDiffColor=24,50,27'
-		'deletionInlineDiffColor=52,27,30'
-		'detailedDeletionInlineDiffColor=74,24,27'
+		'inlineDiffColor=23,51,26'
+		'detailedInlineDiffColor=15,79,17'
+		'deletionInlineDiffColor=77,24,26'
+		'detailedDeletionInlineDiffColor=142,15,17'
 }