Find/Replace overlay: fix content assist not available in Java files

Ctrl+Space in the overlay's search/replace fields never opened content
assist in Java files because ContentAssistCommandAdapter activates its
handler on the workbench-root IHandlerService without an expression,
while the target editor keeps a part-scoped handler for the same command
permanently active via ActivePartExpression. Since handler conflicts are
resolved by evaluated source priority regardless of context nesting, the
editor's higher-priority activation always wins, independent of where
focus actually is.

Fix this by registering an ordinary FindReplaceOverlayAction for the
content-assist command, the same way EDIT_FIND_AND_REPLACE is already
handled: activated through registerAction()'s overlayFocusedExpression,
which alone already outranks the editor's ActivePartExpression-based
activation regardless of which IHandlerService the activation lives
on, so no separate, part-scoped override mechanism is needed for this.

Fixes #2651

Author: HeikoKlare

bundles/org.eclipse.ui.workbench.texteditor/src/org/eclipse/ui/internal/findandreplace/overlay/FindReplaceOverlay.java

@@ -501,6 +501,8 @@ private void createCloseTools() {
 		// Also close on Ctrl+F: otherwise it would reopen (a duplicate of) this very overlay.
 		commandSupport.registerAction(
 				new FindReplaceOverlayAction(this::close, IWorkbenchCommandConstants.EDIT_FIND_AND_REPLACE));
+		commandSupport.registerAction(new FindReplaceOverlayAction(this::triggerContentAssist,
+				ITextEditorActionDefinitionIds.CONTENT_ASSIST_PROPOSALS));
 
 		// Close button
 		new AccessibleToolItemBuilder(closeTools).withStyleBits(SWT.PUSH)
@@ -976,6 +978,14 @@ private void updateContentAssistAvailability() {
 		setContentAssistsEnablement(findReplaceLogic.isAvailableAndActive(SearchOptions.REGEX));
 	}
 
+	private void triggerContentAssist() {
+		if (searchBar.isFocusControl() && contentAssistSearchField.isEnabled()) {
+			contentAssistSearchField.openProposalPopup();
+		} else if (okayToUse(replaceBar) && replaceBar.isFocusControl() && contentAssistReplaceField.isEnabled()) {
+			contentAssistReplaceField.openProposalPopup();
+		}
+	}
+
 	private void decorate() {
 		if (findReplaceLogic.isAvailableAndActive(SearchOptions.REGEX)) {
 			SearchDecoration.validateRegex(getFindString(), searchBarDecoration);