Tweak CalcTextSize() awkward width rounding/ceiling code to reduce floating-point imprecisions altering the result by 1 even at relatively small width. (#791) + apply same fudge factor to less important roundings. ceilf() is still measurable e.g. ballpark +0.5 for 200k calls.

diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index 46ee7cd..1c55358 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt

@@ -55,6 +55,8 @@
 - Fonts:
   - Added `IMGUI_DISABLE_DEFAULT_FONT_BITMAP`/`IMGUI_DISABLE_DEFAULT_FONT_VECTOR` to
     disable embedding either fonts separately. (#9407)
+  - Tweak CalcTextSize() awkward width rounding/ceiling code to reduce floating-point
+    imprecisions altering the result by 1 even at relatively small width. (#791)
 - DrawList:
   - Minor optimization to AddLine(), AddLineH(), AddLineV() functions. (#4091)
 - Demo:

diff --git a/imgui.cpp b/imgui.cpp
index ecd3bc1..735deb9 100644
--- a/imgui.cpp
+++ b/imgui.cpp

@@ -6199,11 +6199,10 @@
     ImVec2 text_size = font->CalcTextSizeA(font_size, FLT_MAX, wrap_width, text, text_display_end, NULL);
 
     // Round
-    // FIXME: This has been here since Dec 2015 (7b0bf230) but down the line we want this out.
-    // FIXME: Investigate using ceilf or e.g.
-    // - https://git.musl-libc.org/cgit/musl/tree/src/math/ceilf.c
-    // - https://embarkstudios.github.io/rust-gpu/api/src/libm/math/ceilf.rs.html
-    text_size.x = IM_TRUNC(text_size.x + 0.99999f);
+    // FIXME: This has been here since Dec 2015 (7b0bf230 then 4622fa4b6) but down the line we want this out. See #791.
+    // Investigate using ceilf or e.g. https://git.musl-libc.org/cgit/musl/tree/src/math/ceilf.c, https://embarkstudios.github.io/rust-gpu/api/src/libm/math/ceilf.rs.html
+    // The problem is that ceilf() has a measurable cost. Not high, but measurable!
+    text_size.x = IM_TRUNC(text_size.x + 0.999f);
 
     return text_size;
 }

diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index 093f0f1..b112f47 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp

@@ -656,7 +656,7 @@
 int ImDrawList::_CalcCircleAutoSegmentCount(float radius) const
 {
     // Automatic segment count
-    const int radius_idx = (int)(radius + 0.999999f); // ceil to never reduce accuracy
+    const int radius_idx = (int)(radius + 0.999f); // ceil to never reduce accuracy
     if (radius_idx >= 0 && radius_idx < IM_COUNTOF(_Data->CircleSegmentCounts))
         return _Data->CircleSegmentCounts[radius_idx]; // Use cached value
     else

diff --git a/imgui_widgets.cpp b/imgui_widgets.cpp
index 3807f1f..a98cb69 100644
--- a/imgui_widgets.cpp
+++ b/imgui_widgets.cpp

@@ -1748,14 +1748,14 @@
     const float separator_thickness = style.SeparatorTextBorderSize;
     const ImVec2 min_size(label_size.x + extra_w + padding.x * 2.0f, ImMax(label_size.y + padding.y * 2.0f, separator_thickness));
     const ImRect bb(pos, ImVec2(window->WorkRect.Max.x, pos.y + min_size.y));
-    const float text_baseline_y = ImTrunc((bb.GetHeight() - label_size.y) * style.SeparatorTextAlign.y + 0.99999f); //ImMax(padding.y, ImTrunc((style.SeparatorTextSize - label_size.y) * 0.5f));
+    const float text_baseline_y = ImTrunc((bb.GetHeight() - label_size.y) * style.SeparatorTextAlign.y + 0.999f); //ImMax(padding.y, ImTrunc((style.SeparatorTextSize - label_size.y) * 0.5f));
     ItemSize(min_size, text_baseline_y);
     if (!ItemAdd(bb, id))
         return;
 
     const float sep1_x1 = pos.x;
     const float sep2_x2 = bb.Max.x;
-    const float seps_y = ImTrunc((bb.Min.y + bb.Max.y) * 0.5f + 0.99999f);
+    const float seps_y = ImTrunc((bb.Min.y + bb.Max.y) * 0.5f + 0.999f);
 
     const float label_avail_w = ImMax(0.0f, sep2_x2 - sep1_x1 - padding.x * 2.0f);
     const ImVec2 label_pos(pos.x + padding.x + ImMax(0.0f, (label_avail_w - label_size.x - extra_w) * style.SeparatorTextAlign.x), pos.y + text_baseline_y); // FIXME-ALIGN
@@ -8952,7 +8952,7 @@
         // Tooltip on hover
         if (hovered && inner_bb.Contains(g.IO.MousePos))
         {
-            const float t = ImClamp((g.IO.MousePos.x - inner_bb.Min.x) / (inner_bb.Max.x - inner_bb.Min.x), 0.0f, 0.9999f);
+            const float t = ImClamp((g.IO.MousePos.x - inner_bb.Min.x) / (inner_bb.Max.x - inner_bb.Min.x), 0.0f, 0.999f);
             const int v_idx = (int)(t * item_count);
             IM_ASSERT(v_idx >= 0 && v_idx < values_count);