Bug 40857: add context parameter for modal overlay

- The combobox dropdown in modals scrolled with the modal body instead of overlaying, due to Bootstrap's overflow: hidden on
modals clipping extending content.
- Added context parameter to combobox config; setting context: 'modal' uses Popper's fixed positioning strategy for proper
overlay.
- For reference: https://popper.js.org/docs/v2/constructors/#strategy

To test:
1. Make an item bookable
   1.1. Go to a record (e.g. http://localhost:8081/cgi-bin/koha/catalogue/detail.pl?biblionumber=285)
   1.2. Go to the Items tab on the left side of the screen
   1.3. Under Bookable, choose Yes and click 'Update'

2. Place a booking
   2.1. Click 'Place booking' at the top of the detailed record screen
   2.2. Fill out the form
   2.3. Click 'Submit'

3. Try to cancel the booking
   3.1. Go to the 'Bookings' tab on the left side of the screen
   3.2. Click 'Cancel' next to the booking
   --> A modal appears with a text field for the cancellation reason
   3.3. Click 'No, do not cancel'

4. Add one authorised value in BOOKING_CANCELLATION
   4.1. Go to Administration > Authorized values
   4.2. Search for BOOKING_CANCELLATION
   4.3. Click 'Add'
   4.4. Fill out the form
   4.5. Click 'Save'

5. Redo step 3
   --> The option appears as a dropdown below the text field -- OK

6. Add one or two more authorised values

7. Redo step 3
   --> The options appear as a dropdown below the text field -- OK

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Martin Renvoize <martin.renvoize@openfifth.co.uk>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

koha-tmpl/intranet-tmpl/prog/en/modules/bookings/list.tt

@@ -432,6 +432,7 @@
             $("#cancellation-reason").comboBox({
                 displayProperty: 'name',
                 placeholder: _("Select or type a reason"),
+                context: 'modal',
             });
 
         });

koha-tmpl/intranet-tmpl/prog/en/modules/circ/circulation.tt

@@ -1250,6 +1250,7 @@
             $("#cancellation-reason").comboBox({
                 displayProperty: 'name',
                 placeholder: _("Select or type a reason"),
+                context: 'modal',
             });
         });
     </script>

koha-tmpl/intranet-tmpl/prog/en/modules/members/moremember.tt

@@ -862,6 +862,7 @@
             $("#cancellation-reason").comboBox({
                 displayProperty: 'name',
                 placeholder: _("Select or type a reason"),
+                context: 'modal',
             });
         });
     </script>

koha-tmpl/intranet-tmpl/prog/js/combobox.js

@@ -11,6 +11,7 @@
      * @param {boolean} [config.useKeyAsValue=false] - Whether to use the option's key (either HTML data-* or JavaScript `valueProperty`) as the value of the input (default: false).
      * @param {string} [config.placeholder='Select or type a value'] - Placeholder text for the input element.
      * @param {string} [config.labelId=''] - Optional ID of the associated label element.
+     * @param {string} [config.context='default'] - Positioning context ('default' or 'modal').
      *
      * @example
      * ```html
@@ -35,9 +36,10 @@
      *     displayProperty: 'name',
      *     valueProperty: 'id',
      *     useKeyAsValue: true,
+     *     context: 'modal',
      * });
      * // or using jQuery
-     * $("#generic-combobox").comboBox({ ... });
+     * $("#generic-combobox").comboBox({ displayProperty: 'name', context: 'modal' });
      * </script>
      * ```
      */
@@ -51,6 +53,7 @@
             placeholder = "Select or type a value",
             labelId = "",
             useKeyAsValue = false,
+            context = "default",
         } = config;
 
         const input = document.getElementById(inputId);
@@ -60,9 +63,16 @@
             return;
         }
 
-        const bootstrapDropdown = new bootstrap.Dropdown(input, {
+        const dropdownOptions = {
             autoClose: false,
-        });
+            popperConfig: {
+                strategy: { default: "absolute", modal: "fixed" }[context],
+            },
+        };
+        const bootstrapDropdown = new bootstrap.Dropdown(
+            input,
+            dropdownOptions
+        );
 
         // Existing options from HTML
         const existingOptions = Array.from(dropdownMenu.querySelectorAll("li"))