Rectangles with square corners are filled with hard edges on the GLES backend, so rectangles that share an edge leave no seam between them

On the GLES backend, Prompt's selection showed a dark line between every two of its rows. `GlesCanvas::fill_rect` always grew the quad by 1 px and passed `u_pad = 1`, so the rect shader never took its solid path, which only runs with no radius, no stroke and no pad, and antialiased every rectangle over a band from −1 to +1 px around its edge. A pixel along an edge, its centre half a pixel inside, was covered at 84 % even when the edge lay on a whole pixel, so two rectangles that met there each left 16 % of what lay beneath showing through, which over a dark background reads as a line. The software backend does not show it, since tiny-skia covers a pixel-aligned edge fully.
A rectangle with square corners now gets no pad, which takes the shader's hard-edged path, as tiny-skia fills it. Rounded rectangles keep the pad and their antialiased edges. A square rectangle whose edges fall between pixels now has a hard edge where it had a soft one.

src/gles_render/primitives.rs

@@ -132,7 +132,8 @@ impl GlesCanvas
 		// straight edges of pills / rounded rects. `u_size` and `u_radii`
 		// stay anchored to the original rect — `u_pad` lets the shader
 		// remap `v_uv` from the larger quad back into rect-local space.
-		let pad = 1.0_f32;
+		// Square corners take the shader's hard-edged path instead, so rects that share an edge leave no seam.
+		let pad = if corners.is_zero() { 0.0_f32 } else { 1.0 };
 		let expanded = rect.expand( pad );
 		let mvp = ortho_rect( self.width, self.height, expanded );
 		let alpha = color.a * self.global_alpha;