[0.83] 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

@@ -1314,13 +1314,35 @@ 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 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 by the
+  // screen DC's real DPI instead. DIPs are 1/96in by definition, so the
+  // DIP<->HIMETRIC leg always uses 96.
+  //
+  // GetDpiForSystem() (user32) reports exactly what GetDeviceCaps(hdc,
+  // LOGPIXELSX/Y) reports for a GetDC(nullptr) screen DC - both return the
+  // system DPI for the caller's DPI-awareness context - so this avoids taking a
+  // Gdi32 dependency for the measurement. RNW already calls GetDpiForSystem
+  // elsewhere in this file. Windows reports square logical DPI for the screen
+  // (LOGPIXELSX == LOGPIXELSY), so one value covers both axes.
+  const UINT hdcDpi = GetDpiForSystem();
   constexpr float HIMETRIC_PER_INCH = 2540.0f;
+  constexpr float DIPS_PER_INCH = 96.0f;
 
+  // Height is deliberately unconstrained. Do NOT scale LONG_MAX into HIMETRIC:
+  // LONG_MAX * 2540 / 96 is ~5.7e10, far outside LONG, and converting an
+  // out-of-range floating-point value back to an integer type is undefined
+  // behavior. LONG_MAX already means "unbounded" to RichEdit.
   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),
+      std::numeric_limits<LONG>::max()};
 
   SIZE naturalSize = {0, 0};
 
@@ -1340,8 +1362,10 @@ 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 / hdcDpi;
+  float contentHeight = static_cast<float>(naturalSize.cy) * DIPS_PER_INCH / hdcDpi;
 
   return {contentWidth, contentHeight};
 }