Disable theming when high contrast mode is enabled #3095

Currently only working for Windows since Linux and Mac do not report
about high contrast mode being enabled/disabled.

Contributes to
#3095

Author: fedejeanne

bundles/org.eclipse.ui.workbench/eclipseui/org/eclipse/ui/internal/dialogs/ViewsPreferencePage.java

@@ -110,6 +110,7 @@ public class ViewsPreferencePage extends PreferencePage implements IWorkbenchPre
 	private ControlDecoration colorFontsDecorator;
 	private ColorsAndFontsTheme currentColorsAndFontsTheme;
 	private Map<String, String> themeAssociations;
+	private boolean highContrastMode;
 
 	private Button themingEnabled;
 	private Button rescaleAtRuntime;
@@ -125,11 +126,15 @@ protected Control createContents(Composite parent) {
 
 		Composite comp = new Composite(parent, SWT.NONE);
 
-		themingEnabled = createCheckButton(comp, WorkbenchMessages.ThemingEnabled, engine != null);
+		highContrastMode = parent.getDisplay().getHighContrast();
+		// Deactivate theming in high contrast mode
+		themingEnabled = createCheckButton(comp, WorkbenchMessages.ThemingEnabled, isThemingPossible());
+		themingEnabled.setEnabled(!highContrastMode);
 
-		// if started with "-cssTheme none", CSS settings should be disabled
-		// but other appearance settings should be *not* disabled
-		if (engine == null) {
+		// if started with "-cssTheme none" or if high contrast mode is active,
+		// CSS settings should be disabled but other appearance settings should be *not*
+		// disabled
+		if (!isThemingPossible()) {
 			GridLayout layout = new GridLayout(1, false);
 			layout.horizontalSpacing = 10;
 			comp.setLayout(layout);
@@ -192,6 +197,21 @@ protected Control createContents(Composite parent) {
 		return comp;
 	}
 
+	/**
+	 * @return <code>true</code> if there is a theme engine set (<i>i.e.</i> if the
+	 *         workbench started with the checkbox "enable theming" set to
+	 *         <code>true</code>) and the <i>high contrast mode</i> is
+	 *         <strong>disabled on the OS.
+	 *
+	 * @implNote Currently only Windows is able to tell if <i>high contrast mode</i>
+	 *           is active. Linux and Mac lack this functionality (they always say
+	 *           it is <b>disabled</b>).
+	 *
+	 */
+	private boolean isThemingPossible() {
+		return engine != null && !highContrastMode;
+	}
+
 	private void createRescaleAtRuntimeCheckButton(Composite parent) {
 		if (!OS.isWindows()) {
 			return;
@@ -313,7 +333,7 @@ public void init(IWorkbench workbench) {
 	public boolean performOk() {
 		IEclipsePreferences prefs = InstanceScope.INSTANCE
 				.getNode(PREF_QUALIFIER_ECLIPSE_E4_UI_WORKBENCH_RENDERERS_SWT);
-		if (engine != null) {
+		if (isThemingPossible()) {
 			ITheme theme = getSelectedTheme();
 			if (theme != null) {
 				engine.setTheme(getSelectedTheme(), true);
@@ -326,9 +346,14 @@ public boolean performOk() {
 		apiStore.setValue(IWorkbenchPreferenceConstants.USE_COLORED_LABELS, useColoredLabels.getSelection());
 
 		prefs.putBoolean(StackRenderer.MRU_KEY, enableMru.getSelection());
-		boolean themingEnabledChanged = prefs.getBoolean(PartRenderingEngine.ENABLED_THEME_KEY, true) != themingEnabled
-				.getSelection();
-		prefs.putBoolean(PartRenderingEngine.ENABLED_THEME_KEY, themingEnabled.getSelection());
+		boolean themingEnabledChanged = false;
+		// Only if the setting is modifiable by the user does checking for it (and
+		// storing it) make sense
+		if (themingEnabled.isEnabled()) {
+			themingEnabledChanged = prefs.getBoolean(PartRenderingEngine.ENABLED_THEME_KEY, true) != themingEnabled
+					.getSelection();
+			prefs.putBoolean(PartRenderingEngine.ENABLED_THEME_KEY, themingEnabled.getSelection());
+		}
 
 		boolean isRescaleAtRuntimeChanged = false;
 		if (rescaleAtRuntime != null) {
@@ -356,7 +381,7 @@ public boolean performOk() {
 		String restartDialogTitle = null;
 		String restartDialogMessage = null;
 
-		if (engine != null) {
+		if (isThemingPossible()) {
 			ITheme theme = getSelectedTheme();
 			boolean themeChanged = theme != null && !theme.equals(currentTheme);
 			boolean colorsAndFontsThemeChanged = !PlatformUI.getWorkbench().getThemeManager().getCurrentTheme().getId()
@@ -417,7 +442,7 @@ private void setColorsAndFontsTheme(ColorsAndFontsTheme theme) {
 	protected void performDefaults() {
 		IEclipsePreferences defaultPrefs = DefaultScope.INSTANCE
 				.getNode(PREF_QUALIFIER_ECLIPSE_E4_UI_WORKBENCH_RENDERERS_SWT);
-		if (engine != null) {
+		if (isThemingPossible()) {
 			setColorsAndFontsTheme(currentColorsAndFontsTheme);
 
 			engine.setTheme(defaultTheme, true);
@@ -441,7 +466,7 @@ protected void performDefaults() {
 
 	@Override
 	public boolean performCancel() {
-		if (engine != null) {
+		if (isThemingPossible()) {
 			setColorsAndFontsTheme(currentColorsAndFontsTheme);
 
 			if (currentTheme != null && !currentTheme.equals(engine.getActiveTheme())) {