Remove round tabs support from CTabRendering

[vogella]

Summary

• Removes the CSS-based rounded tab rendering feature (swt-corner-radius) from CTabRendering
• Deletes CSSPropertyCornerRadiusSWTHandler and removes its plugin.xml registration
• Removes setCornerRadius() from ICTabRendering interface
• Removes USE_ROUND_TABS preference constant, the preference checkbox in ViewsPreferencePage, and all related message strings
• Simplifies rendering methods (drawTabBody, drawSelectedTab, drawUnselectedTab) to square-corner-only paths, removing the CirclePart enum and circle-drawing methods

Closes #3806

Test plan

• Verify tabs render correctly (square corners) in the default theme
• Verify the "Use round tabs" checkbox is gone from Preferences > General > Appearance
• Verify no compilation errors in affected bundles

🤖 Generated with Claude Code

[eclipse-platform-bot]

This pull request changes some projects for the first time in this development cycle.
Therefore the following files need a version increment:

bundles/org.eclipse.e4.ui.css.swt/META-INF/MANIFEST.MF

An additional commit containing all the necessary changes was pushed to the top of this PR's branch. To obtain these changes (for example if you want to push more changes) either fetch from your fork or apply the git patch.

Git patch

From 341f7df2fd6ed15a66d4889561862f0d9a4ec1cd Mon Sep 17 00:00:00 2001
From: Eclipse Platform Bot <platform-bot@eclipse.org>
Date: Sun, 29 Mar 2026 19:28:58 +0000
Subject: [PATCH] Version bump(s) for 4.40 stream


diff --git a/bundles/org.eclipse.e4.ui.css.swt/META-INF/MANIFEST.MF b/bundles/org.eclipse.e4.ui.css.swt/META-INF/MANIFEST.MF
index 01e69485e3..dcdb5e262b 100644
--- a/bundles/org.eclipse.e4.ui.css.swt/META-INF/MANIFEST.MF
+++ b/bundles/org.eclipse.e4.ui.css.swt/META-INF/MANIFEST.MF
@@ -1,7 +1,7 @@
 Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-SymbolicName: org.eclipse.e4.ui.css.swt;singleton:=true
-Bundle-Version: 0.16.0.qualifier
+Bundle-Version: 0.16.100.qualifier
 Bundle-Name: %pluginName
 Bundle-Vendor: %providerName
 Bundle-Localization: plugin
-- 
2.53.0

Further information are available in Common Build Issues - Missing version increments.

[github-actions]

Test Results

   852 files  ±0     852 suites  ±0   58m 55s ⏱️ + 7m 0s
 7 884 tests ±0   7 641 ✅ ±0  243 💤 ±0  0 ❌ ±0 
20 154 runs  ±0  19 498 ✅ ±0  656 💤 ±0  0 ❌ ±0 

Results for commit c9c7459. ± Comparison against base commit 157e7ed.

♻️ This comment has been updated with latest results.

[vogella]

Dark theme with this change:

Light theme with this change:

[BeckerWdf]

Dark theme with this change:
Light theme with this change:

This looks familiar to me. How did the version before look like?

[vogella]

The same (screenshots prove that nothing broke)

[BeckerWdf]

The same (screenshots prove that nothing broke)

For user's that have set the now removed setting, the UI will change significantly. We should document this in the N&N.

[HeikoKlare]

Nice code simplification coming with the removal of rounded tabs support. Thank you.

[HeikoKlare]

In my opinion, this kind of change should have a minor version bump (at least in the og.eclipse.e4.ui.workbench.renderers.swt, maybe also in the org.eclipse.e4.ui.css.swt one).

[vogella]

Done