Windows: adjust default ClipRect to better match rendering of thick borders. (#7887, #7888 + #3312, #7540, #3756, #6170, #6365)

docs/CHANGELOG.txt

@@ -46,6 +46,9 @@ Other changes:
 - IO, InputText: fixed an issue where typing text in a InputText() would defer character
   processing by one frame, because of the trickling input queue. Reworked interleaved
   keys<>char trickling to take account for keys known to input characters. (#7889, #4921, #4858)
+- Windows: adjust default ClipRect to better match rendering of thick borders (which are in
+  theory not supported). Compensate for the fact that borders are centered around the windows
+  edge rather than inner. (#7887, #7888 + #3312, #7540, #3756, #6170, #6365)
 - MultiSelect+TreeNode+Drag and Drop: fixed an issue where carrying a drag and drop
   payload over an already open tree node would incorrectly select it. (#7850)
 - MultiSelect+TreeNode: default open behavior is OpenOnDoubleClick + OpenOnArrow

imgui.cpp

@@ -7066,10 +7066,14 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         // Affected by window/frame border size. Used by:
         // - Begin() initial clip rect
         float top_border_size = (((flags & ImGuiWindowFlags_MenuBar) || !(flags & ImGuiWindowFlags_NoTitleBar)) ? style.FrameBorderSize : window->WindowBorderSize);
-        window->InnerClipRect.Min.x = ImFloor(0.5f + window->InnerRect.Min.x + window->WindowBorderSize);
-        window->InnerClipRect.Min.y = ImFloor(0.5f + window->InnerRect.Min.y + top_border_size);
-        window->InnerClipRect.Max.x = ImFloor(0.5f + window->InnerRect.Max.x - window->WindowBorderSize);
-        window->InnerClipRect.Max.y = ImFloor(0.5f + window->InnerRect.Max.y - window->WindowBorderSize);
+
+        // Try to match the fact that our border is drawn centered over the window rectangle, rather than inner.
+        // This is why we do a *0.5f here. We don't currently even technically support large values for WindowBorderSize,
+        // see e.g #7887 #7888, but may do after we move the window border to become an inner border (and then we can remove the 0.5f here).
+        window->InnerClipRect.Min.x = ImFloor(0.5f + window->InnerRect.Min.x + window->WindowBorderSize * 0.5f);
+        window->InnerClipRect.Min.y = ImFloor(0.5f + window->InnerRect.Min.y + top_border_size * 0.5f);
+        window->InnerClipRect.Max.x = ImFloor(window->InnerRect.Max.x - window->WindowBorderSize * 0.5f);
+        window->InnerClipRect.Max.y = ImFloor(window->InnerRect.Max.y - window->WindowBorderSize * 0.5f);
         window->InnerClipRect.ClipWithFull(host_rect);
 
         // Default item width. Make it proportional to window size if window manually resizes