Thanks for the PR.

There is a printf() left.

Is the value of io.DisplayFramebufferScale & returned by Platform_GetWindowFramebufferScale then correct on Emscripten on all 3 backends?

There is a printf() left.

fixed, sorry

Is the value of io.DisplayFramebufferScale & returned by Platform_GetWindowFramebufferScale then correct on Emscripten on all 3 backends?

• example_sdl2_opengl3:

  • ContentScale is equal to 1 (OK)
  • DisplayFrameBufferScale is equal to 2 on a retina screen, and varies according to browser zoom: OK

• example_glfw_opengl3 and example_glfw_webgpu:

  • ContentScale is equal to 1 (OK)
  • DisplayFrameBufferScale is now correctly handled (and depends on the browser zoom level): OK

This was done using the flag --use-port=contrib.glfw3 instead of -s USE_GLFW=3

This way, we use emscripten-glfw, which offers a better support for HighDPI.

• https://emscripten.org/docs/compiling/Contrib-Ports.html
• https://github.com/pongasoft/emscripten-glfw

Note: an alternative (but faulty) solution would have been to stick to -sUSE_GLFW=3 and then modify main.cpp by adding :

#ifdef __EMSCRIPTEN__
glfwWindowHint(GLFW_SCALE_TO_MONITOR, GLFW_TRUE);
#endif

This solution works partially (the FrameBufferScale is passed). However, it will create a window with a bad size as soon as the browser zoom level is not 100%. It is therefore not applicable.

I have merged this as 18dca11 + ed7d965 as this is a definitive improvement over 1.92.0 status.

HOWEVER,
FramebufferScale was never designed to be non-integer. As thus, I am not even sure this is working correctly in term of font quality.

A way to compare would be to use a nicer font, display text in varying sizes, capture and do a visual compare between e.g. native macOS and Emscripten. Does it looks the same? Could someone perform this test? (perhaps display ImGui::GetFontSize() + ImGui::GetCurrentContext()->FontRasterizerDensity on screen next to tests to ensure matching values are used in both tests)
Using imgui_freetype would facilitate the compare because hinting output more pixels precisely to the pixel grid, and if there is a difference it would be much easier to spot and compare.

One current benefit of FramebufferScale over ContentScale, is that currently the earlier works as-is, without application needing to support scaling. vs ContentScale requires Style scaling of sizes for which we don't currently provide a dynamic solution.

If we notice the output of non-integer FramebufferScale values is not perfect, or not good enough, we could perfectly decide to do the opposite on those platforms where FramebufferScale is not integer: move this scaling factor from FramebufferScale to ContentScale. This would put the same burden for scaling on app side as on Windows.

I am away at the moment (no access to my desktop), but unless somebody else take a look at it in the meantime, I should be able to run a test like this, this week-end/early next week.

Same as @ypujante , I can test this next week.

Below are several screenshots on macOS + emscripten (firefox and chrome) at various zoom levels.
FBS_shots.zip

Images content:

emscripten_chrome_fbs1.5.png
emscripten_chrome_fbs2.0.png
emscripten_chrome_fbs2.5.png
emscripten_chrome_fbs3.5.png
emscripten_firefox_fbs1.58.png
emscripten_firefox_fbs2.0.png
emscripten_firefox_fbs2.22.png
emscripten_firefox_fbs2.61.png
emscripten_firefox_fbs3.33.png
emscripten_firefox_fbs7.5.png
macOS_fbs2.0.png

Example:

This uses imgui_freetype + PlutoSvg and the font DroidSans (ImGui Bundle default).

Code (for my own reference if additional tests and/or changes are needed)

// This example will use the default settings of HelloImGui: imgui_freetype + plutosvg, font droid_sans.ttf
#include "hello_imgui/hello_imgui.h"
#include "imgui.h"
#include "imgui_internal.h"

void Gui() {
    auto fbs = ImGui::GetIO().DisplayFramebufferScale;
    ImGui::Text("Framebuffer scale: %.2f, %.2f", fbs.x, fbs.y);

    for (float font_size = 8.0f; font_size <= 32.0f; font_size += 2.0f) {
        ImGui::PushFont(nullptr, font_size);
        ImGui::Text("Font size: %.2f, FontRasterizerDensity=%.1f  the quick brown fox jumps over the lazy dog", font_size, ImGui::GetCurrentContext()->FontRasterizerDensity);
        ImGui::PopFont();
        ImGui::Separator();
    }
}

int main() { HelloImGui::Run(Gui); }

@ocornut did you need more than the test that @pthom created? It seems pretty complete to me, but I am not sure if this is what you were looking for.