GC.textExtent() ignores requested font family and measures wider text in headless (Xvfb/GTK) vs. a real win32 display

[thebraun2fw-max]

Title: GC.textExtent() ignores requested font family and measures wider text in headless (Xvfb/GTK) vs. a real win32 display

Summary

Using a minimal, standalone SWT snippet (no Eclipse RCP application, no GEF/draw2d — pure org.eclipse.swt.graphics.GC#textExtent()), the same font name/size/text produces different measured extents when run headless on Linux under Xvfb (GTK) vs. on a real Windows display (win32).

Two distinct problems are visible in the data:

1. Font family name has no effect on measurement in the headless/GTK case. Requesting "Segoe UI", "DejaVu Sans", and "Sans" (all at 9pt) produces byte-identical width/height for every test string under Xvfb/GTK — even though Segoe UI (a physically installed TTF, verified via fc-match/fc-list outside this test) is a genuinely different typeface from DejaVu Sans. On win32, the three names produce measurably different results, as expected (Segoe UI differs from the DejaVu Sans/Sans fallback).
2. Headless/GTK measurements are systematically wider than the win32 equivalent for the same font size, by roughly 6%–33% depending on the string (see table below). This causes downstream layout problems in any Eclipse RCP application that pre-computes fixed pixel geometry from text measured headlessly and expects it to match what was authored/measured on a real (typically Windows) display — e.g. GEF/draw2d figures with fixed bounds, as used by Archi (archimatetool/archi) for diagram rendering.

Environment

• SWT jar (Linux): org.eclipse.swt.gtk.linux.x86_64_3.134.0.v20260515-1429.jar
• SWT jar (Windows): org.eclipse.swt.win32.win32.x86_64_3.126.0.v20240528-0813.jar
  (version mismatch between the two — noted as a caveat below, but the within-platform font-name-has-no-effect finding on Linux is version-independent)
• Linux: Docker container, Xvfb virtual display (xvfb-run -a ...), JRE 21
• Windows: normal desktop session, real display, JRE (same major version)
• Font Segoe UI verified present on the Linux side (fc-match "Segoe UI" correctly resolves to segoeui.ttf, confirmed via fc-list)

Reproduction

Minimal snippet (attached in full below) does, per font name in {"Segoe UI", "DejaVu Sans", "Sans"}:

Display display = new Display();
Image dummy = new Image(display, 10, 10);
GC gc = new GC(dummy);
Font font = new Font(display, fontName, 9, SWT.NORMAL);
gc.setFont(font);
Point extent = gc.textExtent(text);
System.out.println(fontName + " -> " + extent.x + "x" + extent.y + " for: " + text);

Run headless:

xvfb-run -a java -cp ".:$SWT_JAR" TextExtentTest

Run on Windows desktop (real display, no Xvfb):

java -cp ".;<swt.jar>" TextExtentTest

Results

All measurements at 9pt. width in pixels as returned by GC.textExtent().

Text	Linux/Xvfb/GTK (all 3 font names identical)	Windows/win32, Segoe UI	Windows/win32, DejaVu Sans/Sans	Linux vs Windows Segoe UI
Компонент	70	62	63	+12.9%
финансово-экономического	174	160	159	+8.75%
ОООС «Ванильный Бюджет» (новая версия)	277	243	255	+14.0%
топик 20 \nзапрос на формирование УГПП по	215	193	200	+11.4%
192.168.231.150 \ngoogle.com	112	84	96	+33.3%
Hello World	67	63	64	+6.3%
AAAAAAAAAAAAAAAAAAAA (20×A)	179	160	140	+11.9%

Note on the Linux side: all three requested font names produced these exact same numbers, row for row — i.e. the "Segoe UI" column, "DejaVu Sans" column, and "Sans" column values on Linux are one and the same, whereas on Windows they genuinely differ per font.

Why this matters

Any application that measures/lays out text headlessly (CI pipelines, automated report/diagram generation, etc.) using SWT GC.textExtent() and expects that layout to match what a user sees/authored on a real (typically Windows) desktop will get incorrect, generally too-wide-but-not-reliably-scaled text metrics, and — separately — will get no differentiation at all between different font families in the headless/GTK case. We hit this in practice via Archi (archimatetool/archi) generating HTML diagram reports headlessly in a CI/Docker pipeline: text overflows figure boundaries that were sized correctly on Windows desktop, and no combination of installing/selecting fonts on the Linux side changed the outcome — which this snippet now explains: the font selection wasn't taking effect on the measurement path at all.

Caveats

• The two SWT jar versions compared (3.134.0 Linux vs 3.126.0 Windows) are not identical, so version drift could contribute to the magnitude of the width difference. However, the font-name-has-no-effect-on-Linux finding is internally consistent within the single Linux run (three different names, one identical result) and doesn't depend on cross-version comparison at all.
• fc-match/fc-list outside this JVM confirm Segoe UI is present and correctly resolvable at the fontconfig level — so the lack of differentiation appears to be happening either in SWT's GTK backend itself, or in how it (or possibly Pango/Cairo under Xvfb) selects a font for measurement, not in fontconfig/system-level font registration.

Attached

• TextExtentTest.java (full source, self-contained, no dependencies beyond the platform SWT jar)
• Raw output: linux_headless_output.txt, windows_output.txt

Happy to run further variations (different SWT versions on both sides, real X11 display on Linux instead of Xvfb, etc.) if that would help narrow this down further.

windows_output.txt

linux_headless_output.txt
TextExtentTest.java