enable folding with "Only show selected Java element"

[danthe1st]

Fixes #2264
Depends on eclipse-platform/eclipse.platform.ui#3074 (Is it necessary to change the version requirements in some way even if I am not depending on any new API?)
Depends on #2410 for this to work correctly with extended folding That dependency PR has been merged.

I think this should be tested with JDT-UI (with this PR) before eclipse-platform/eclipse.platform.ui#3074 is merged.

What it does

This PR enables folding when Window > Preferences > Java > Editor > Only show the selected Java element is enabled.

This depends on eclipse-platform/eclipse.platform.ui#3074 to work correctly.

How to test

• Include Allow using visible regions with projections eclipse-platform/eclipse.platform.ui#3074 not necessary any more since that's merged, just make sure to use the latest I-build of eclipse.platform.ui
• Enable Window > Preferences > Java > Editor > Only show the selected Java element
• Enable folding in Window > Preferences > Java > Editor > Folding
• Open a Java file
• Folding should work
• Select an element in the outline
• Observe that only the selected element is shown in the editor.
• Folding should still work
• All of that should work with and without extended folding or custom folding regions. Editing the file shouldn't break it.

Author checklist

• I have thoroughly tested my changes
  • If only a selected region is shown and one enters text at the end of that region (if the curser is exactly after the closing }), everything after the selected region is displayed. I am not sure whether that's an issue. See https://github.com/user-attachments/assets/18cc621f-1c69-45e6-a7ce-97d609f489fe This was an issue with my changes in eclipse.platform.ui which should be fixed.
  • I did not do any performance evaluation.
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[danthe1st]

I wrote some basic tests for the PR in eclipse.platform.ui. Is it also necessary to add integration tests in JDT-UI for this change?

[iloveeclipse]

I wrote some basic tests for the PR in eclipse.platform.ui. Is it also necessary to add integration tests in JDT-UI for this change?

If possible, regression test for original problem?

[danthe1st]

If possible, regression test for original problem?

I added a test that ensures folding is active (even though I am not sure whether this is actually a bug I'm fixing or more implementing a feature/enhancement).

But note that in order to work correctly, the other PR would need to be merged/integrated first (and my test wouldn't detect that currently).

[danthe1st]

The test failure should be because this depends on eclipse-platform/eclipse.platform.ui#3074 which is not merged.

[danthe1st]

The changes I made to DefaultJavaFoldingStructureProvider (making sure the IJavaElement is present) are necessary because it uses the IJavaElement to determine which folding regions are equal. Without this, it tries to update the existing folding projection annotations in a way that caused issues with my change (I think it tried to set existing folding regions to other ones).

[danthe1st]

In situations where #2327 occurs and that region is added/removed when editing the editor, the parts after the visible region can become visible but that's fixed by that PR.

Other than that, I think this PR and eclipse-platform/eclipse.platform.ui#3074 are ready for someone to take a look at it.

[danthe1st]

Would @iloveeclipse or someone else care to review these two PRs at some point?

[iloveeclipse]

Seem to work fine except the binary types (e.g. java.lang.String etc), I guess similar change like it was in JavaEditor is needed for ClassFileEditor.

[danthe1st]

Seem to work fine except the binary types (e.g. java.lang.String etc), I guess similar change like it was in JavaEditor is needed for ClassFileEditor.

Did the "Only show the selected Java element" option ever work with the Class File Editor? I didn't see that doing anything without my change and this PR doesn't seem to impact that.

[iloveeclipse]

Seem to work fine except the binary types (e.g. java.lang.String etc), I guess similar change like it was in JavaEditor is needed for ClassFileEditor.

Did the "Only show the selected Java element" option ever work with the Class File Editor? I didn't see that doing anything without my change and this PR doesn't seem to impact that.

Honestly, no idea, I never used this "Only show the selected Java element" option, just assumed it should work there too :-)
So if it wasn't ther ebefore, don't care!

[jjohnstn]

Hi @danthe1st I'd be happy to review this once the platform UI patch is in.

[danthe1st]

Even though the JDT-UI platform-UI patch is in and the regressions caused by it have been fixed, it is still possible that it gets reverted if it causes more issues so it might be better to see and keep this for 2026-03. Alternatively, it could also be reviewed now.

[iloveeclipse]

Even though the JDT-UI patch is in

You probably mean Platform UI patch?

Alternatively, it could also be reviewed now.

This would be better tha patch here is small enough and shouldn't be a problem to revert.

[danthe1st]

You probably mean Platform UI patch?

Yes, my fault (edited)

This would be better tha patch here is small enough and shouldn't be a problem to revert.

I see. @jjohnstn would you still want to review this now?

[danthe1st]

@jjohnstn ^ Sorry I originally messed up the ping

[danthe1st]

^ rebased, fixed merge conflicts

[jjohnstn]

I am seeing a issue when testing. If I have:

public class TestFolding {

	// region

	void test() {
		// region A
		// endregion A

	}
	void test2() {

	}

	void test2a() {

	}
	void test3() {}

	// endregion

	// region2

	void test4() {

	}

	// endregion2
}

When I select test() in the outline view, it shows test() only but it also shows the // region comment above it (AST elements get tied to line comments above them and on the same line). It is recognized as a custom folding comment but clicking on it does not hide the test() method. It adds the end-of-line stuff in a normal folding on all of the test() lines. I can confirm that folding within test() works fine.

[jjohnstn]

See main-line comment

[danthe1st]

When I select test() in the outline view, it shows test() only but it also shows the // region comment above it (AST elements get tied to line comments above them and on the same line). It is recognized as a custom folding comment but clicking on it does not hide the test() method. It adds the end-of-line stuff in a normal folding on all of the test() lines. I can confirm that folding within test() works fine.

When writing the Platform-UI patch, I ensured that it is not possible to fold anything outside of the visible region (as it didn't make sense IMO). I think I can provide another PR there that ensures that these annotations are hidden.

[danthe1st]

I created eclipse-platform/eclipse.platform.ui#3423 to fix this issue.

[danthe1st]

@jjohnstn Would you like to review this now for 2026-03? There are still eclipse-platform/eclipse.platform.ui#3457 (small GUI change), eclipse-platform/eclipse.platform.ui#3456 (fixing some exception) and eclipse-platform/eclipse.platform.ui#3585 (fixing too much being hidden at the end with Windows line endings) but I think these shouldn't block this review (much).

[jjohnstn]

Looks good.