don't modify topIndex in handleTextChanged if control has not the focus

[tobiasmelcher]

fix split screen issue eclipse-platform/eclipse.platform.ui#2142 by not setting StyledText#topIndex if the control has not the focus.

[tobiasmelcher]

@mickaelistria would you prefer that we delete the complete if-block "if (!isFixedLineHeight() && topIndex > firstLine) {" or is the approach of this pull request also fine?

[github-actions]

Test Results

   480 files   -  3     480 suites   - 3   8m 31s ⏱️ - 4m 3s
 4 060 tests  - 35   4 052 ✅  - 33   4 💤  - 3  4 ❌ +1 
16 138 runs   - 35  16 044 ✅  - 36  87 💤  - 3  7 ❌ +4 

For more details on these failures, see this check.

Results for commit c12914b. ± Comparison against base commit e6588c2.

This pull request removes 35 tests.

org.eclipse.swt.graphics.ImageWin32Tests ‑ testImageShouldHaveDimesionAsPerZoomLevel
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testByteArrayTransfer
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testFileTransfer
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testHtmlTransfer
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testImageTransfer_fromCopiedImage
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testImageTransfer_fromImage
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testImageTransfer_fromImageData
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testImageTransfer_fromImageDataFromImage
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testRtfTransfer
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testTextTransfer
…

[vogella]

We should try to delete the code that we do not know why it is there (eclipse-platform/eclipse.platform.ui#2142) early in the next release cycle to find if this causes any yet unknown issues.

[mickaelistria]

This PR looks good.

We should try to delete the code that we do not know why it is there

Here I think the reason for this code are clear enough: it can happen while editing some lines of non fixed height that the current position of the topIndex doesn't allow to see the whole first edited line. So I think it's best to keep it as proposed.