From a9efe0ca4c26b675c587224e4a23eea2f9372f6d Mon Sep 17 00:00:00 2001
From: CJ van den Berg <cj@vdbonline.com>
Date: Mon, 30 Mar 2026 20:13:10 +0200
Subject: [PATCH] fix(gui): eliminate row clipping at top of window

---
 src/gui/gpu/builtin.glsl.zig | 7 ++++---
 src/gui/gpu/gpu.zig          | 7 ++++---
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/gui/gpu/builtin.glsl.zig b/src/gui/gpu/builtin.glsl.zig
index 61a58294..88fb7501 100644
--- a/src/gui/gpu/builtin.glsl.zig
+++ b/src/gui/gpu/builtin.glsl.zig
@@ -14,6 +14,7 @@ pub const FsParams = extern struct {
     cell_size_y: i32,
     col_count: i32,
     row_count: i32,
+    viewport_height: i32,
 };
 
 const vs_src =
@@ -32,6 +33,7 @@ const fs_src =
     \\uniform int cell_size_y;
     \\uniform int col_count;
     \\uniform int row_count;
+    \\uniform int viewport_height;
     \\uniform sampler2D glyph_tex_glyph_smp;
     \\uniform usampler2D cell_tex_cell_smp;
     \\out vec4 frag_color;
@@ -47,10 +49,8 @@ const fs_src =
     \\
     \\void main() {
     \\    // Convert gl_FragCoord (bottom-left origin) to top-left origin.
-    \\    // row_count * cell_size_y >= viewport height (we ceil the division),
-    \\    // so this formula maps the top screen pixel to row 0.
     \\    int px = int(gl_FragCoord.x);
-    \\    int py = row_count * cell_size_y - 1 - int(gl_FragCoord.y);
+    \\    int py = viewport_height - 1 - int(gl_FragCoord.y);
     \\    int col = px / cell_size_x;
     \\    int row = py / cell_size_y;
     \\
@@ -99,6 +99,7 @@ pub fn shaderDesc(backend: sg.Backend) sg.ShaderDesc {
             desc.uniform_blocks[0].glsl_uniforms[1] = .{ .type = .INT, .glsl_name = "cell_size_y" };
             desc.uniform_blocks[0].glsl_uniforms[2] = .{ .type = .INT, .glsl_name = "col_count" };
             desc.uniform_blocks[0].glsl_uniforms[3] = .{ .type = .INT, .glsl_name = "row_count" };
+            desc.uniform_blocks[0].glsl_uniforms[4] = .{ .type = .INT, .glsl_name = "viewport_height" };
 
             // Glyph atlas texture: R8 → sample_type = FLOAT
             desc.views[0].texture = .{
diff --git a/src/gui/gpu/gpu.zig b/src/gui/gpu/gpu.zig
index 40816fea..b1965e0a 100644
--- a/src/gui/gpu/gpu.zig
+++ b/src/gui/gpu/gpu.zig
@@ -20,7 +20,7 @@ const Rgba8 = gui_cell.Rgba8;
 const log = std.log.scoped(.gpu);
 
 // Maximum glyph atlas dimension.  4096 is universally supported and gives
-// 65536+ glyph slots at typical cell sizes — far more than needed in practice.
+// 65536+ glyph slots at typical cell sizes - far more than needed in practice.
 const max_atlas_dim: u16 = 4096;
 
 fn getAtlasCellCount(cell_size: XY(u16)) XY(u16) {
@@ -339,8 +339,8 @@ pub fn paint(
     top: u16,
     cells: []const Cell,
 ) void {
-    const shader_col_count: u16 = @intCast(@divTrunc(client_size.x + font.cell_size.x - 1, font.cell_size.x));
-    const shader_row_count: u16 = @intCast(@divTrunc(client_size.y + font.cell_size.y - 1, font.cell_size.y));
+    const shader_col_count: u16 = @intCast(@divTrunc(client_size.x, font.cell_size.x));
+    const shader_row_count: u16 = @intCast(@divTrunc(client_size.y, font.cell_size.y));
 
     const copy_col_count: u16 = @min(col_count, shader_col_count);
     const blank_glyph_index = state.generateGlyph(font, ' ', .single);
@@ -422,6 +422,7 @@ pub fn paint(
         .cell_size_y = font.cell_size.y,
         .col_count = shader_col_count,
         .row_count = shader_row_count,
+        .viewport_height = @intCast(client_size.y),
     };
     sg.applyUniforms(0, .{
         .ptr = &fs_params,