Support relative font sizes and px in the CSS engine

[vogella]

The CSS engine now understands em, %, larger and smaller for font-size, resolved against the font the widget inherited. A theme can therefore ask for a slightly larger font without overriding the size the user configured, which was not expressible before.

px was accepted already but silently read as a point size. It is now converted with 1px = 0.75pt, matching the CSS definition of a pixel as 1/96 inch against SWT's 1/72 inch font height. Style sheets that relied on the old reading render smaller and have to state pt, as the cssbridge example now does. Unitless values and any other unit keep being read as points.

The font shorthand accepts the new forms too, a relative size on a FontDefinition scales the registered font instead of replacing it, and docs/CSS.md documents the supported values.

[github-actions]

Test Results

   858 files  ±  0     858 suites  ±0   1h 6m 33s ⏱️ + 26m 5s
 8 305 tests + 26   8 062 ✅ + 26  243 💤 ± 0  0 ❌ ±0 
20 784 runs  +209  20 114 ✅ +176  670 💤 +33  0 ❌ ±0 

Results for commit f8c7a09. ± Comparison against base commit 9560649.

♻️ This comment has been updated with latest results.

[Copilot]

🟡 Changes recommended

Relative sizing compounds on reapply, and shorthand keyword classification is incorrect.

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Pull request overview

Adds relative font sizing and correct CSS pixel-to-point conversion to the SWT CSS engine.

Changes:

• Supports em, %, larger, smaller, and corrected px sizing.
• Extends shorthand parsing, tests, documentation, and examples.
• Updates existing fixtures to use explicit point sizes.

File summaries

File	Review
tests/org.eclipse.e4.ui.tests.css.swt/src/org/eclipse/e4/ui/tests/css/swt/ShellTest.java	Updates fixtures to explicit point units.
tests/org.eclipse.e4.ui.tests.css.swt/src/org/eclipse/e4/ui/tests/css/swt/LabelTest.java	Tests new size forms and shorthand behavior.
tests/org.eclipse.e4.ui.tests.css.swt/src/org/eclipse/e4/ui/tests/css/swt/CTabFolderTest.java	Updates fixtures to explicit point units.
tests/org.eclipse.e4.ui.tests.css.swt/src/org/eclipse/e4/ui/tests/css/swt/ButtonTest.java	Updates fixtures to explicit point units.
tests/org.eclipse.e4.ui.tests.css.swt/src/org/eclipse/e4/ui/css/swt/helpers/CSSSWTFontHelperTest.java	Tests conversion and relative sizing.
examples/org.eclipse.e4.demo.cssbridge/css/common.css	Preserves example sizing with pt.
docs/CSS.md	Documents supported font-size values.
bundles/org.eclipse.e4.ui.css.swt/src/org/eclipse/e4/ui/css/swt/helpers/CSSSWTFontHelper.java	Moderate: Relative sizes compound during style reapplication instead of using the inherited baseline.
bundles/org.eclipse.e4.ui.css.core/src/org/eclipse/e4/ui/css/core/css2/CSS2FontHelper.java	Moderate: Shorthand parsing misclassifies quoted and mixed-case larger/smaller values.

Review details

Suppressed comments (2)

bundles/org.eclipse.e4.ui.css.swt/src/org/eclipse/e4/ui/css/swt/helpers/CSSSWTFontHelper.java:220

• Relative sizes make font conversion depend on oldFontData, but the engine caches converted fonts by the CSS properties alone (CSSEngineImpl.java:1083-1092 and CSSResourcesHelpers.java:79-81). Two widgets with the same family/style and font-size: 200% but inherited heights of 10 and 20 therefore share the first cached font instead of resolving to 20 and 40. Include the inherited height in the cache identity for relative sizes, or bypass caching for these context-dependent conversions.

			case EM -> scaleFontHeight(numeric.value(), oldFontData);
			case PERCENT -> scaleFontHeight(numeric.value() / 100, oldFontData);

bundles/org.eclipse.e4.ui.css.swt/src/org/eclipse/e4/ui/css/swt/helpers/CSSSWTFontHelper.java:241

• When called from CSSPropertyFontDefinitionHandler, oldFontData is definition.getValue(), and the scaled result is immediately written back with setValue. Reapplying the same larger, smaller, em, or % rule therefore scales the previous override again (for example, 10pt becomes 12pt and then 14pt), rather than consistently scaling the registered baseline. Preserve and use the unstyled definition value as the relative-size base.

	private static OptionalInt scaleFontHeight(double factor, FontData oldFontData) {
		if (oldFontData == null) {
			return OptionalInt.empty();
		}
		return OptionalInt.of(toFontHeight(oldFontData.getHeight() * factor));

• Files reviewed: 9/9 changed files
• Comments generated: 2
• Review effort level: Balanced

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