fix(kit): improve caret management for InputNumber on step action

projects/cdk/utils/dom/value-binding.ts

@@ -19,7 +19,21 @@ export function tuiValueBinding(
     const el = tuiInjectElement<HTMLInputElement>();
 
     effect(() => {
+        if (el.value === value()) {
+            return;
+        }
+
+        const {selectionStart, selectionEnd} = el;
+
         el.value = value();
+
+        if (el.matches(':focus')) {
+            /**
+             * After programmatic updates of input's value, caret is automatically placed at the end –
+             * revert to the previous position
+             */
+            el.setSelectionRange(selectionStart, selectionEnd);
+        }
     });
 
     return value;

projects/demo-playwright/tests/kit/input-number/input-number.pw.spec.ts

@@ -382,6 +382,96 @@
                     });
                 });
             });
+
+            describe('caret position on step action', () => {
+                beforeEach(async ({page}) => {
+                    await tuiGoto(page, `${DemoRoute.InputNumber}/API?step=1&postfix=kg`);
+
+                    await expect(inputNumber.textfield).toHaveValue('');
+                    await expect(inputNumber.textfield).not.toBeFocused();
+                });
+
+                test('Empty unfocused textfield => Click + => Textfield is focused & Caret is placed before postfix', async () => {
+                    await inputNumber.stepUp.click();
+
+                    await expect(inputNumber.textfield).toHaveValue('1kg');
+                    await expect(inputNumber.textfield).toHaveJSProperty(
+                        'selectionStart',
+                        1,
+                    );
+                    await expect(inputNumber.textfield).toHaveJSProperty(
+                        'selectionEnd',
+                        1,
+                    );
+                });
+
+                test('Focused textfield with postfix only => Press ArrowDown => Caret is placed before postfix', async () => {
+                    await inputNumber.textfield.focus();
+                    await expect(inputNumber.textfield).toHaveValue('kg');
+                    await inputNumber.textfield.press('ArrowDown');
+
+                    await expect(inputNumber.textfield).toHaveValue(`${CHAR_MINUS}1kg`);
+                    await expect(inputNumber.textfield).toHaveJSProperty(
+                        'selectionStart',
+                        2,
+                    );
+                    await expect(inputNumber.textfield).toHaveJSProperty(
+                        'selectionEnd',
+                        2,
+                    );
+                });
+
+                describe('Keeps caret position on step', () => {
+                    beforeEach(async () => {
+                        await inputNumber.textfield.fill('42');
+
+                        await expect(inputNumber.textfield).toHaveValue('42kg');
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionStart',
+                            2,
+                        );
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionEnd',
+                            2,
+                        );
+                        await inputNumber.textfield.press('ArrowLeft');
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionStart',
+                            1,
+                        );
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionEnd',
+                            1,
+                        );
+                    });
+
+                    test('via button', async () => {
+                        await inputNumber.stepUp.click();
+                        await expect(inputNumber.textfield).toHaveValue('43kg');
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionStart',
+                            1,
+                        );
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionEnd',
+                            1,
+                        );
+                    });
+
+                    test('via keyboard arrow', async () => {
+                        await inputNumber.textfield.press('ArrowUp');
+                        await expect(inputNumber.textfield).toHaveValue('43kg');
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionStart',
+                            1,
+                        );
+                        await expect(inputNumber.textfield).toHaveJSProperty(
+                            'selectionEnd',
+                            1,
+                        );
+                    });
+                });
+            });
         });
 
         describe('[prefix] & [postfix] props', () => {

projects/kit/components/input-number/input-number.component.ts

@@ -202,11 +202,17 @@ export class TuiInputNumber extends TuiControl<number | null> {
     }
 
     protected onStep(step: number): void {
-        this.textfieldValue.set(
-            this.formatNumber(
-                tuiClamp((this.value() ?? 0) + step, this.min(), this.max()),
-            ),
+        const newValue = this.formatNumber(
+            tuiClamp((this.value() ?? 0) + step, this.min(), this.max()),
         );
+
+        if (this.value() === null) {
+            const caretIndex = newValue.length - this.postfix().length;
+
+            setTimeout(() => this.element.setSelectionRange(caretIndex, caretIndex));
+        }
+
+        this.textfieldValue.set(newValue);
     }
 
     private formatNumber(value: number | null): string {