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Updated the PR to address the review comments:

• Made the Yoga prev/next button width match the painted rounded widths:

  fcitx5/src/ui/classic/inputwindow.cpp

  Lines 1045 to 1047 in bc071e2

  	YGNodeStyleSetWidth(buttonNode_.get(),
  	roundedScaledMetric(prev.width(), scale) +
  	roundedScaledMetric(next.width(), scale));

• Rounded natural-size tile dimensions instead of truncating scaled values:

  fcitx5/src/ui/classic/theme.cpp

  Lines 334 to 343 in bc071e2

  	if (height < 0) {
  	height = static_cast<int>(
  	std::lround(scaledMetric(resizeHeight, targetScale) +
  	targetMarginTop + targetMarginBottom));
  	}
  	if (width < 0) {
  	width = static_cast<int>(
  	std::lround(scaledMetric(resizeWidth, targetScale) +
  	targetMarginLeft + targetMarginRight));

• Scaled shadow margins used for XCB input window positioning:

  fcitx5/src/ui/classic/xcbinputwindow.cpp

  Lines 80 to 115 in bc071e2

  	const auto scale = metricScale();
  	int leftSW = roundedScaledMetric(
  	theme.inputPanel->shadowMargin->marginLeft.value(), scale);
  	int rightSW = roundedScaledMetric(
  	theme.inputPanel->shadowMargin->marginRight.value(), scale);
  	int actualWidth = width() - leftSW - rightSW;
  	actualWidth = actualWidth <= 0 ? width() : actualWidth;
  	if (closestScreen != nullptr) {
  	int newX = std::max(x, closestScreen->left());
  	if (newX + actualWidth > closestScreen->right()) {
  	newX = closestScreen->right() - actualWidth;
  	}
  	x = std::max(newX, closestScreen->left());
  	}
  	// exclude shadow border width
  	x -= leftSW;
  	return x;
  	}
  	int XCBInputWindow::calculatePositionY(const Rect &cursorRect,
  	const Rect *closestScreen) const {
  	// TODO: RTL support.
  	int y = cursorRect.top();
  	auto &theme = parent_->theme();
  	const auto scale = metricScale();
  	int topSW = roundedScaledMetric(
  	theme.inputPanel->shadowMargin->marginTop.value(), scale);
  	int bottomSW = roundedScaledMetric(
  	theme.inputPanel->shadowMargin->marginBottom.value(), scale);

• Rebuilt the blur region when the DPI scale changes, not only when the window size changes:

  fcitx5/src/ui/classic/xcbinputwindow.cpp

  Lines 179 to 237 in bc071e2

  	void XCBInputWindow::updateBlurRegion(int width, int height, double scale) {
  	if (!atomBlur_) {
  	return;
  	}
  	blurScale_ = scale;
  	Rect rect(0, 0, width, height);
  	shrink(rect, *ui_->parent()->theme().inputPanel->blurMargin, scale);
  	if (!*ui_->parent()->theme().inputPanel->enableBlur || rect.isEmpty()) {
  	xcb_delete_property(ui_->connection(), wid_, atomBlur_);
  	return;
  	}
  	std::vector<uint32_t> data;
  	if (ui_->parent()->theme().inputPanel->blurMask->empty()) {
  	data.push_back(rect.left());
  	data.push_back(rect.top());
  	data.push_back(rect.width());
  	data.push_back(rect.height());
  	} else {
  	auto region = parent_->theme().mask(parent_->theme().maskConfig(),
  	width, height, scale);
  	for (const auto &rect : region) {
  	data.push_back(rect.left());
  	data.push_back(rect.top());
  	data.push_back(rect.width());
  	data.push_back(rect.height());
  	}
  	}
  	xcb_change_property(ui_->connection(), XCB_PROP_MODE_REPLACE, wid_,
  	atomBlur_, XCB_ATOM_CARDINAL, 32, data.size(),
  	data.data());
  	}
  	void XCBInputWindow::update(InputContext *inputContext) {
  	if (!wid_) {
  	return;
  	}
  	auto oldVisible = visible();
  	if (inputContext) {
  	updateDPI(inputContext);
  	}
  	auto [width, height] = InputWindow::update(inputContext);
  	if (!visible()) {
  	if (oldVisible) {
  	xcb_unmap_window(ui_->connection(), wid_);
  	hoverIndex_ = -1;
  	}
  	return;
  	}
  	const auto scale = metricScale();
  	bool sizeChanged = width != this->width() || height != this->height();
  	bool blurScaleChanged = atomBlur_ && scale != blurScale_;
  	if (sizeChanged) {
  	resize(width, height);
  	}
  	if (sizeChanged || blurScaleChanged) {
  	updateBlurRegion(width, height, scale);

• Used the bottom highlight margin for XCB menu item region height (this looks like an existing bug in the original code):

  fcitx5/src/ui/classic/xcbmenu.cpp

  Lines 511 to 517 in bc071e2

  	.setSize(
  	maxItemWidth +
  	roundedScaledMetric(*highlightMargin.marginLeft, scale) +
  	roundedScaledMetric(*highlightMargin.marginRight, scale),
  	maxItemHeight +
  	roundedScaledMetric(*highlightMargin.marginTop, scale) +
  	roundedScaledMetric(*highlightMargin.marginBottom, scale));

The change could mostly be minimized to xcb part if you just simply downscale the X11 event coordinates and it shall be similar to wayland model.

Thanks for the suggestion. I refactored the patch following this approach.

Theme layout and Pango metrics now stay in logical coordinates. The XCB backend uses Cairo device scale for rendering, scales only X11-specific window geometry and regions, and converts X11 pointer coordinates back to logical coordinates before passing them to the shared UI code.

Changed the fixed 40-pixel offsets to physicalFromLogical(40).