fix(textinput): center single-line text correctly at >100% display scale

change/react-native-windows-fix-textinput-centering-dpi.json

@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "Fix single-line TextInput text/caret sitting low and bottom-clipped at >100% display scale (GetContentSize DPI normalization)",
+  "packageName": "react-native-windows",
+  "email": "collindanielschneide@gmail.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/Fabric/Composition/TextInput/WindowsTextInputComponentView.cpp

@@ -55,6 +55,12 @@ std::string BstrToStdString(BSTR bstr, int cp = CP_UTF8) {
   return str;
 }
 
+// GetDeviceCaps (used in GetContentSize to normalize RichEdit's device-pixel
+// measurements to DIPs) is exported from Gdi32. Microsoft.ReactNative does not
+// otherwise reference Gdi32, so link it explicitly to avoid an unresolved
+// external symbol (LNK2019) in builds that don't already pull it in transitively.
+#pragma comment(lib, "Gdi32.lib")
+
 MSO_CLASS_GUID(ITextHost, "13E670F4-1A5A-11cf-ABEB-00AA00B65EA1") // IID_ITextHost
 MSO_CLASS_GUID(ITextServices, "8D33F740-CF58-11CE-A89D-00AA006CADC5") // IID_ITextServices
 MSO_CLASS_GUID(ITextServices2, "8D33F741-CF58-11CE-A89D-00AA006CADC5") // IID_ITextServices2
@@ -1314,13 +1320,25 @@ std::pair<float, float> WindowsTextInputComponentView::GetContentSize() const no
 
   // Use the layout width as the constraint (always multiline)
   float availableWidth = m_layoutMetrics.frame.size.width;
-  float scale = m_layoutMetrics.pointScaleFactor;
-  float dpi = m_layoutMetrics.pointScaleFactor * GetDpiForSystem();
+
+  // TxGetNaturalSize measures in the *device pixels of `hdc`*. GetDC(nullptr)
+  // returns a screen DC whose logical DPI (GetDeviceCaps LOGPIXELS) is the system
+  // DPI - typically 96 - and is unrelated to the per-monitor display scale carried
+  // in pointScaleFactor. The previous code conflated the two
+  // (dpi = pointScaleFactor * GetDpiForSystem()) and then divided the returned
+  // natural size by pointScaleFactor, so at any display scale > 100% the measured
+  // content height came back short by exactly that scale and
+  // calculateContentVerticalOffset() over-centered the text (parked low /
+  // bottom-clipped). Normalize both conversions by the DC's real DPI instead. DIPs
+  // are 1/96in by definition, so the DIP<->HIMETRIC leg always uses 96.
+  const int hdcDpiX = GetDeviceCaps(hdc, LOGPIXELSX);
+  const int hdcDpiY = GetDeviceCaps(hdc, LOGPIXELSY);
   constexpr float HIMETRIC_PER_INCH = 2540.0f;
+  constexpr float DIPS_PER_INCH = 96.0f;
 
   SIZE extentHimetric = {
-      static_cast<LONG>(availableWidth * scale * HIMETRIC_PER_INCH / dpi),
-      static_cast<LONG>(std::numeric_limits<LONG>::max() * HIMETRIC_PER_INCH / dpi)};
+      static_cast<LONG>(availableWidth * HIMETRIC_PER_INCH / DIPS_PER_INCH),
+      static_cast<LONG>(std::numeric_limits<LONG>::max() * HIMETRIC_PER_INCH / DIPS_PER_INCH)};
 
   SIZE naturalSize = {0, 0};
 
@@ -1340,8 +1358,10 @@ std::pair<float, float> WindowsTextInputComponentView::GetContentSize() const no
     return {0.0f, 0.0f};
   }
 
-  float contentWidth = static_cast<float>(naturalSize.cx) / scale;
-  float contentHeight = static_cast<float>(naturalSize.cy) / scale;
+  // naturalSize is in the DC's device pixels; convert device px -> DIPs using the
+  // DC's actual DPI (px * 96 / hdcDpi), not pointScaleFactor.
+  float contentWidth = static_cast<float>(naturalSize.cx) * DIPS_PER_INCH / hdcDpiX;
+  float contentHeight = static_cast<float>(naturalSize.cy) * DIPS_PER_INCH / hdcDpiY;
 
   return {contentWidth, contentHeight};
 }