Scale inner-context font rasterizer density by the host's display density

src/context_wrapper.h

@@ -214,6 +214,13 @@ class ContainedContext
     bool m_anyItemActive = false;
     bool m_hovered = false;
 
+    // Font rasterizer density of the *host* context, sampled in begin() right
+    // after BeginChild() (i.e. after ImGui has set it from the host viewport's
+    // FramebufferScale). 1.0 on a standard-DPI display, 2.0 on a typical HiDPI
+    // one. The inner context has no platform window of its own, so ImGui can
+    // only ever derive 1.0 for it -- we have to carry the host's value across.
+    float m_hostFontDensity = 1.f;
+
     float m_scale = m_config.default_zoom, m_scaleTarget = m_config.default_zoom;
     ImVec2 m_scroll = {0.f, 0.f};
 };
@@ -227,10 +234,19 @@ inline ContainedContext::~ContainedContext()
 // In begin(), this is called twice: once for the outer context's child window
 // (so the outer renderer rasterizes at the correct density), and once inside
 // the inner context's Begin() when extra_window_wrapper is enabled.
+//
+// The density we want is (host display density * canvas zoom): the zoom factor
+// alone is only correct on a 1x display. The inner context is created without a
+// platform backend, so its viewport FramebufferScale is 0 and its
+// DisplayFramebufferScale is 1 -- ImGui's own SetCurrentWindow() therefore
+// derives a density of 1.0 for it no matter what the host runs at. Multiplying
+// in the host's density keeps node text as crisp as the rest of the app on
+// HiDPI displays.
 inline void ContainedContext::setFontDensity()
 {
 #if IMGUI_VERSION_NUM >= 19198
-    ImGui::SetFontRasterizerDensity(roundf(m_scale * 100.0f) / 100.0f); // Round density to two digits.
+    const float density = m_hostFontDensity * m_scale;
+    ImGui::SetFontRasterizerDensity(roundf(density * 100.0f) / 100.0f); // Round density to two digits.
 #endif
 }
 
@@ -239,6 +255,14 @@ inline void ContainedContext::begin()
     ImGui::PushID(this);
     ImGui::PushStyleColor(ImGuiCol_ChildBg, m_config.color);
     ImGui::BeginChild("view_port", m_config.size, 0, ImGuiWindowFlags_NoMove);
+    // BeginChild() -> SetCurrentWindow() has just reset the density from the
+    // host viewport's FramebufferScale, so this reads the host's true display
+    // density. Sample it before setFontDensity() overwrites it.
+#if IMGUI_VERSION_NUM >= 19198
+    m_hostFontDensity = ImGui::GetFontRasterizerDensity();
+    if (m_hostFontDensity <= 0.f)
+        m_hostFontDensity = 1.f;
+#endif
     // Set font density on the OUTER context's child window so the outer renderer
     // rasterizes fonts at the correct scale before we switch context below.
     setFontDensity();