Terminal area renders clipped to a sub-rectangle on Wayland fractional scaling (1.25, 1.5, …)

[manuacl]

Bug

Under KDE Plasma 6 Wayland on a fractional-scale output (1.25, 1.5, …), the embedded Ghostty terminal area renders into the bottom-left corner of an oversized framebuffer instead of filling the GLArea. The visible terminal occupies roughly 1 / (fractional_scale)² of the pane; keystrokes are accepted but the text is drawn outside the visible region.

At integer scales (1.0, 2.0) the bug does not occur.

Screenshots (before / after) to follow in a comment — gh issue create doesn't accept image uploads.

Reproduction

1. KDE Plasma 6 Wayland (tested on Bazzite Fedora 44, single output at 2048×1152)
2. System Settings → Display → set scale to 1.25 (or 1.5)
3. Launch the Limux AppImage without GDK_BACKEND=x11 (the =x11 env var works around the bug by routing through XWayland, where KWin bitmap-composites the whole window)
4. Open any workspace

Expected: the terminal fills the GLArea, text renders at the correct size for the fractional scale.

Actual: terminal occupies the bottom-left ~62% (at 1.25) / ~44% (at 1.5) of the pane and the rest is black. Typing is consumed but appears outside the visible area.

Root cause

refresh_surface_display in rust/limux-host-linux/src/terminal.rs passes:

• ghostty_surface_set_size(w, h) where w, h are the GLArea's logical (CSS) allocation, and
• ghostty_surface_set_content_scale(scale, scale) where scale = gl_area.scale_factor() (integer).

GTK4 allocates the GLArea's backing FBO at logical_size × scale_factor() physical pixels. On Wayland fractional outputs, the compositor downscales that integer-scaled buffer to the fractional surface size via wp-fractional-scale-v1. Ghostty's renderer treats set_size as the physical-pixel canvas and set_content_scale only for HiDPI font/cell metrics, so passing logical pixels leaves it drawing into the bottom-left sub-rectangle of the oversized FBO.

Workaround

GDK_BACKEND=x11 in the launcher — XWayland reports an integer scale and KWin handles the fractional downscale via bitmap composite. Functional but loses native Wayland behavior and exposes a separate symbolic-icon issue (#80).

Environment

• Bazzite (Fedora 44), KDE Plasma 6 Wayland, NVIDIA open driver 595.71.05
• Limux v0.1.19 (built from main AppImage with the SVG-loader patch from Pane action toolbar: 4 of 6 icons render as broken-image fallback (Fedora/Bazzite, v0.1.19 AppImage) #80)
• GTK 4.22.4 system, libghostty bundled

PR with fix to follow.