Problems with GLSL 1.2 floor () in Vertex Shader

Question

Problems with GLSL 1.2 floor () in Vertex Shader

I am trying to calculate texture coordinates based on the coordinates of an incoming vertex in Vertex Shader. This is a stripped down version of my attempt:

#version 120

varying vec4 color;

uniform sampler2D heightmap;
uniform ivec2 heightmapSize;

void main(void)
{
   vec2 fHeightmapSize = vec2(heightmapSize);
   vec2 pos = gl_Vertex.zx + vec2(0.5f, 0.5f);
   vec2 offset = floor(fHeightmapSize * pos) + vec2(0.5f, 0.5f);

   if (fract(offset.x) > 0.45f && fract(offset.x) < 0.55f 
       && fract(offset.y) > 0.45f && fract(offset.y) < 0.55f)
      color = vec4(0.0f, 1.0f, 0.0f, 1.0f);
   else
      color = vec4(1.0f, 0.0f, 0.0f, 1.0f);

   // gl_Position = ...
   // ...
}

gl_Vertex is in [-0.5, 0.5]^2, on the XZ plane. So what I'm basically trying to do is

first create a float vec2 from heightmapSize ivec2 that contains the width and height of the sample height of the map.
Then I convert the coordinates of the vertices to an interval [0, 1]^2.
Then I calculate the offset in the texture coordinates, multiplying the vertex position by the height size. The left side (using floor()) should return the number of texels in each direction.
: Texure 4x4, (0.55, 0.5). , 2 , → (2.0, 2.0).
(0.5, 0.5), . (2.0, 2.0) (2.5, 2.5). :, , 0.5 .