Setting window size to a non-integer value can make the window move by itself

[ldecarufel]

Calling ImGui::SetWindowSize() with a size that hasn't been rounded to an integer value can make the window move by itself due to rounding errors inside ImGui::Begin().

Let's consider the following call:
ImGui::SetWindowSize(ImVec2(348.48f, 400.0f));

This will set the value of "window->SizeFull.x" to "348.480011" in the SetWindowSize() function (the values were taken from the debugger).

On the next call to ImGui::Begin(), when the window position is clamped to the visible area, a rounding error can make the window position change (see my comments in the code):

// Clamp position so it stays visible
if (!(flags & ImGuiWindowFlags_ChildWindow))
{
    if (!window_pos_set_by_api && window->AutoFitFramesX <= 0 && window->AutoFitFramesY <= 0 && g.IO.DisplaySize.x > 0.0f && g.IO.DisplaySize.y > 0.0f) // Ignore zero-sized display explicitly to avoid losing positions if a window manager reports zero-sized window when initializing or minimizing.
    {
        ImVec2 padding = ImMax(style.DisplayWindowPadding, style.DisplaySafeAreaPadding);
        // Here window->Pos.x == 170.000000 and window->Size.x == 348.480011
        window->Pos = ImMax(window->Pos + window->Size, padding) - window->Size;
        // Here window->Pos.x == 169.999969 due to rounding error
        window->Pos = ImMin(window->Pos, g.IO.DisplaySize - padding);
    }
}

window->Pos = ImFloor(window->Pos);
// And finally, here window->Pos.x has become 169.000000

The solution would be to round the window size in the ImGui::SetWindowSize() function.

Of course, I changed the code on our side to make sure the size was always rounded before calling ImGui::SetWindowSize(), but it would be safer to round the size in the ImGui function to avoid weird behavior.

[ocornut]

Thank you Louis. Should be fixed now!

[przemkovv]

I found that the same issue appears when SizeConstraintRect has non rounded values. In that case, the CalcSizeAfterConstraint function returns a new, non-rounded size which is then used by a code fragment quoted by OP.

// g.NextWindowData.SizeConstraintRect = {Min={x=475.200012 y=0.000000000 } Max={x=475.200012 y=688.799988 } }
// new_size = {x=475.000000 y=491.000000 }

ImRect cr = g.NextWindowData.SizeConstraintRect;
new_size.x = (cr.Min.x >= 0 && cr.Max.x >= 0) ? ImClamp(new_size.x, cr.Min.x, cr.Max.x) : window->SizeFull.x;
new_size.y = (cr.Min.y >= 0 && cr.Max.y >= 0) ? ImClamp(new_size.y, cr.Min.y, cr.Max.y) : window->SizeFull.y;

// new_size = {x=475.200012 y=491.000000 }