matplotlib · timhoffm · Mar 11, 2026 · Feb 11, 2026 · Feb 11, 2026 · Feb 11, 2026
diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
@@ -270,14 +270,24 @@ def _update_device_pixel_ratio(self, event=None):
         elif sys.platform == "linux":
             ratio = self._tkcanvas.winfo_fpixels('1i') / 96
         if ratio is not None and self._set_device_pixel_ratio(ratio):
-            # The easiest way to resize the canvas is to resize the canvas
-            # widget itself, since we implement all the logic for resizing the
-            # canvas backing store on that event.
+            # Resize the canvas widget, then explicitly update the figure
+            # size to match the actual widget dimensions.  When the canvas
+            # is constrained by a geometry manager (pack/grid), <Configure>
+            # may not fire after configure(), so we handle the resize
+            # directly — similar to Qt's _update_pixel_ratio approach.
             w, h = self.get_width_height(physical=True)
             self._tkcanvas.configure(width=w, height=h)
+            self._resize_figure_for_canvas_size(
+                self._tkcanvas.winfo_width(),
+                self._tkcanvas.winfo_height())
 
     def resize(self, event):
-        width, height = event.width, event.height
+        self._resize_figure_for_canvas_size(event.width, event.height)
+
+    def _resize_figure_for_canvas_size(self, width, height):
+        """Update figure size and redraw based on a given canvas size."""
+        if width <= 0 or height <= 0:
+            return
 
         # compute desired figure size in inches
         dpival = self.figure.dpi

diff --git a/lib/matplotlib/tests/test_backend_tk.py b/lib/matplotlib/tests/test_backend_tk.py
@@ -4,6 +4,7 @@
 import platform
 import subprocess
 import sys
+from unittest.mock import patch
 
 import pytest
 
@@ -280,3 +281,52 @@ def test_figure(master):
                        foreground="white")
     test_figure(root)
     print("success")
+
+
+@_isolated_tk_test(success_count=1)
+def test_dpi_change_triggers_resize():
+    """
+    Test that _update_device_pixel_ratio recalculates figure.size_inches
+    using the actual widget dimensions, so the render size matches the
+    visible canvas area even when constrained by a layout manager.
+    See issue #31126.
+    """
+    import tkinter as tk
+    from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg
+    from matplotlib.figure import Figure
+
+    root = tk.Tk()
+    root.geometry("400x300")
+    root.update_idletasks()
+
+    fig = Figure(dpi=100)
+    fig.add_subplot(111)
+
+    canvas = FigureCanvasTkAgg(fig, master=root)
+    canvas.get_tk_widget().pack(fill=tk.BOTH, expand=True)
+    canvas.draw()
+    root.update_idletasks()
+
+    actual_w = canvas.get_tk_widget().winfo_width()
+    actual_h = canvas.get_tk_widget().winfo_height()
+    assert actual_w > 0 and actual_h > 0
+
+    # Simulate a 2x DPI change through _update_device_pixel_ratio.
+    # Mock the platform-specific DPI query to return ratio=2.0.
+    with patch.object(sys, 'platform', 'linux'), \
+         patch.object(canvas._tkcanvas, 'winfo_fpixels',
+                      return_value=192.0):
+        canvas._update_device_pixel_ratio()
+
+    # Verify the render size matches the actual widget size, NOT the
+    # inflated physical size from get_width_height(physical=True).
+    size = fig.get_size_inches()
+    render_w = round(size[0] * fig.dpi)
+    render_h = round(size[1] * fig.dpi)
+    assert abs(render_w - actual_w) <= 2, \
+        f"render width {render_w} != actual width {actual_w}"
+    assert abs(render_h - actual_h) <= 2, \
+        f"render height {render_h} != actual height {actual_h}"
+
+    print("success")
+    root.destroy()