Snap the clip mask and fix bounds checking

webrender/res/brush.glsl

@@ -91,6 +91,7 @@ void main(void) {
 #ifdef WR_FEATURE_ALPHA_PASS
     write_clip(
         vi.world_pos,
+        vi.snap_offset,
         clip_area
     );
 #endif

webrender/res/prim_shared.glsl

@@ -222,9 +222,9 @@ VertexInfo write_transform_vertex(RectWithSize local_segment_rect,
     return vi;
 }
 
-void write_clip(vec4 world_pos, ClipArea area) {
+void write_clip(vec4 world_pos, vec2 snap_offset, ClipArea area) {
     vec2 uv = world_pos.xy * uDevicePixelRatio +
-        world_pos.w * (area.common_data.task_rect.p0 - area.screen_origin);
+        world_pos.w * (snap_offset + area.common_data.task_rect.p0 - area.screen_origin);
     vClipMaskUvBounds = vec4(
         area.common_data.task_rect.p0,
         area.common_data.task_rect.p0 + area.common_data.task_rect.size
@@ -243,15 +243,17 @@ float do_clip() {
     // anything outside of the mask is considered transparent
     //Note: we assume gl_FragCoord.w == interpolated(1 / vClipMaskUv.w)
     vec2 mask_uv = vClipMaskUv.xy * gl_FragCoord.w;
-    bvec4 inside = lessThanEqual(
-        vec4(vClipMaskUvBounds.xy, mask_uv),
-        vec4(mask_uv, vClipMaskUvBounds.zw));
+    bvec2 left = lessThanEqual(vClipMaskUvBounds.xy, mask_uv); // inclusive
+    bvec2 right = greaterThan(vClipMaskUvBounds.zw, mask_uv); // non-inclusive
     // bail out if the pixel is outside the valid bounds
-    if (!all(inside)) {
+    if (!all(bvec4(left, right))) {
         return 0.0;
     }
     // finally, the slow path - fetch the mask value from an image
-    ivec3 tc = ivec3(mask_uv, vClipMaskUv.z);
+    // Note the Z getting rounded to the nearest integer because the variable
+    // is still interpolated and becomes a subject of precision-caused
+    // fluctuations, see https://bugzilla.mozilla.org/show_bug.cgi?id=1491911
+    ivec3 tc = ivec3(mask_uv, vClipMaskUv.z + 0.5);
     return texelFetch(sCacheA8, tc, 0).r;
 }
 

webrender/res/ps_split_composite.glsl

@@ -80,6 +80,7 @@ void main(void) {
 
     write_clip(
         world_pos,
+        vec2(0.0),
         clip_area
     );
 

webrender/res/ps_text_run.glsl

@@ -229,7 +229,7 @@ void main(void) {
     vec2 f = (vi.local_pos - glyph_rect.p0) / glyph_rect.size;
 #endif
 
-    write_clip(vi.world_pos, clip_area);
+    write_clip(vi.world_pos, vi.snap_offset, clip_area);
 
     switch (color_mode) {
         case COLOR_MODE_ALPHA: