Skip gradient image for flat look in FormHeading and CSS handler

[vogella]

Summary

• FormHeading (setTextBackground): Detect when all gradient colors are identical and use setBackground(color) instead of generating a gradient image via FormImages. This avoids unnecessary Image allocation and GC rendering for flat themes.
• CSSPropertyFormHandler: Simplify identical-color gradient lists from CSS into a single-color array before passing to the form widget, preventing unnecessary gradient processing downstream.

These changes implement Levels 1 and 4 from #3803, complementing the Level 2/3 changes in #3807.

Fixes #3803

Test plan

• Verify flat theme renders correctly with solid background (no gradient image created)
• Verify gradient themes still render correctly with distinct colors
• Verify CSS text-background-color with identical gradient stops uses solid background
• Verify CSS text-background-color with distinct gradient stops still creates gradient

🤖 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.ui.forms/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 02b42986fd5dbe0f2d3a2783c01646e42aca2c37 Mon Sep 17 00:00:00 2001
From: Eclipse Platform Bot <platform-bot@eclipse.org>
Date: Fri, 20 Mar 2026 13:54:30 +0000
Subject: [PATCH] Version bump(s) for 4.40 stream


diff --git a/bundles/org.eclipse.ui.forms/META-INF/MANIFEST.MF b/bundles/org.eclipse.ui.forms/META-INF/MANIFEST.MF
index 22e1a339c7..b17c669780 100644
--- a/bundles/org.eclipse.ui.forms/META-INF/MANIFEST.MF
+++ b/bundles/org.eclipse.ui.forms/META-INF/MANIFEST.MF
@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: %name
 Bundle-SymbolicName: org.eclipse.ui.forms;singleton:=true
-Bundle-Version: 3.14.0.qualifier
+Bundle-Version: 3.14.100.qualifier
 Bundle-Vendor: %provider-name
 Bundle-Localization: plugin
 Export-Package: org.eclipse.ui.forms,
-- 
2.53.0

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

[github-actions]

Test Results

   852 files  ± 0     852 suites  ±0   56m 13s ⏱️ - 1m 41s
 7 873 tests + 6   7 630 ✅ + 6  243 💤 ±0  0 ❌ ±0 
20 145 runs  +18  19 489 ✅ +18  656 💤 ±0  0 ❌ ±0 

Results for commit 50e467e. ± Comparison against base commit 1446276.

♻️ This comment has been updated with latest results.

[vogella]

Rebased onto eclipse/master and resolved merge conflicts in FlatLookTest.java. Fixed a bug in the FormHeading optimization where the background image wasn't being cleared correctly in all cases. Verified with mvn clean verify in forms test bundle.