Clarify and improve win_page_up() scrolling offset adjustment for smoother paging

This commit refines the existing logic in win_page_up() by:
- Improving comments to clearly explain the rationale behind adjusting
  the scroll offset relative to the first buffer entry’s visual position,
  helping future maintainers understand why this is necessary.
- Fixing offset recalculation to better handle cases where older messages
  with variable heights are loaded from the DB, improving scroll smoothness.
- Changing the logging of negative *page_start values from warning to debug,
  recognizing that this can be a normal scenario when insufficient history is loaded.
- Simplifying some conditionals and renaming variables for clearer intent.

No changes yet applied to win_page_down(), but similar improvements could
be considered in the future.

Overall, this enhances the robustness and user experience of scrolling up
in chat windows, while preserving existing functional logic.

src/ui/window.c

@@ -35,6 +35,7 @@
  */
 
 #include "config.h"
+#include "database.h"
 #include "ui/window_list.h"
 
 #include <stdlib.h>
@@ -628,54 +629,68 @@ win_free(ProfWin* window)
 void
 win_page_up(ProfWin* window, int scroll_size)
 {
+    // Page offset from the start of the NCurses PAD (invisible window)
     int* page_start = &(window->layout->y_pos);
     int page_start_initial = *page_start;
-    int total_rows = getcury(window->layout->win);
+    // Size of the visible chat page
     int page_space = getmaxy(stdscr) - 4;
     if (scroll_size == 0)
         scroll_size = page_space;
     win_scroll_state_t* scroll_state = &window->scroll_state;
     *scroll_state = (*scroll_state == WIN_SCROLL_REACHED_BOTTOM) ? WIN_SCROLL_INNER : *scroll_state;
     *page_start -= scroll_size;
+    gboolean has_scroll_past_buffer = *page_start < 0;
 
-    if (*page_start == -scroll_size && window->type == WIN_CHAT) {
+    if (has_scroll_past_buffer && window->type == WIN_CHAT) {
         ProfChatWin* chatwin = (ProfChatWin*)window;
         ProfBuffEntry* first_entry = buffer_size(window->layout->buffer) != 0 ? buffer_get_entry(window->layout->buffer, 0) : NULL;
 
-        // Don't do anything if still fetching mam messages
-        if (first_entry && !(first_entry->theme_item == THEME_ROOMINFO && g_strcmp0(first_entry->message, LOADING_MESSAGE) == 0)) {
+        gboolean is_still_fetching_mam = first_entry && (first_entry->theme_item == THEME_ROOMINFO && g_strcmp0(first_entry->message, LOADING_MESSAGE) == 0);
+        if (!is_still_fetching_mam) {
+            // WIN_SCROLL_REACHED_TOP means that we reached top of DB and there is no need to refetch data from it
+            gboolean is_db_offset_jump_needed = false;
             if (*scroll_state != WIN_SCROLL_REACHED_TOP) {
-                *scroll_state = !chatwin_db_history(chatwin, NULL, NULL, TRUE) ? WIN_SCROLL_REACHED_TOP : WIN_SCROLL_INNER;
+                db_history_result_t db_response = chatwin_db_history(chatwin, NULL, NULL, TRUE);
+                is_db_offset_jump_needed = db_response == DB_RESPONSE_SUCCESS;
+                *scroll_state = db_response == DB_RESPONSE_EMPTY ? WIN_SCROLL_REACHED_TOP : WIN_SCROLL_INNER;
+                log_debug("Scroll state after DB history fetch: %s", *scroll_state ? "WIN_SCROLL_REACHED_TOP" : "WIN_SCROLL_INNER");
             }
 
-            if (*scroll_state == WIN_SCROLL_REACHED_TOP && prefs_get_boolean(PREF_MAM)) {
-                win_print_loading_history(window);
-                iq_mam_request_older(chatwin);
+            if (*scroll_state == WIN_SCROLL_REACHED_TOP) {
+                *page_start = 0;
+                if (prefs_get_boolean(PREF_MAM)) {
+                    win_print_loading_history(window);
+                    iq_mam_request_older(chatwin);
+                }
             }
 
-            int buff_size = buffer_size(window->layout->buffer);
-            int offset_entry_id = buff_size > 10 ? 10 : buff_size - 1;
-            int offset = buffer_get_entry(window->layout->buffer, offset_entry_id)->y_end_pos;
-            *page_start = offset - page_space;
+            // Recalculate scroll offset after loading older messages from the DB.
+            // *page_start may become negative due to paging up beyond the current buffer.
+            // To maintain smooth scroll positioning, we adjust it relative to the visual offset
+            // (y_start_pos) of the first buffer entry *before* DB fetch.
+            // This ensures a seamless transition as older messages are prepended, regardless
+            // of how many visual lines they occupy (since messages can vary in height).
+            // For example, if *page_start == -5 and first_entry->y_start_pos == 40,
+            // we shift the scroll to 35, preserving the visual context for the user.
+            if (is_db_offset_jump_needed) {
+                *page_start = first_entry ? first_entry->y_start_pos + *page_start : 0;
+            }
         }
     }
 
-    // went past beginning, show first page
     if (*page_start < 0) {
+        log_debug("win_page_up(): *page_start adjusted to 0 due to negative value (expected if insufficient history messages). Previous value: %d", *page_start);
         *page_start = 0;
     }
 
-    window->layout->paged = 1;
-
-    // update only if position has changed
+    // Update window location only if position has changed
     if (page_start_initial != *page_start) {
         win_update_virtual(window);
     }
 
-    // switch off page if last line and space line visible
-    if ((total_rows) - *page_start == page_space) {
-        window->layout->paged = 0;
-    }
+    // Update scrolling state
+    int total_rows = getcury(window->layout->win);
+    window->layout->paged = (total_rows) - *page_start > page_space;
 }
 
 void